LS1Steve

2004 CTS-V, 77k on car.

I have a noise I'm trying to track down which seems to be getting worse. It sounds like a knocking sound coming from the transmission. It is like a tap that occurs with every RPM, and is in sync with the engine rpm not drive shaft rpm. Can feel it in the floor all over between by feet and the shifter area, louder when car is warmed up.

Rear end is perfect (and again, sound occurs in sync w engine side)

Engine has been replaced 3k ago (Jasper rebuild), same noise existed before and after the new engine.

LS7 clutch and light flywheel installed 7k ago and driven lightly.

Creative Steel engine and trans mounts swapped in 7k ago.

New factory dampener swapped in 7k ago.

New driveshaft with billet couplers just recently put in, no change to this noise before and after (I thought it was a carrier bearing on the 2piece stock driveshaft)

Car accelerates normally, the noise exists from 0-3200rpm regardless of load (louder with load). After 3200 it nearly disappears and smooths out. With clutch in, or when out of gear (clutch in or out) the engine revs perfectly smooth with no abnormal noise of vibration.

With the new engine (and noise existing prior), driveshaft swapped, I though for sure this was a a bent trans input shaft. Had an incident where the flywheel broke and caused major vibration prior to all of the replacements. Sent Trans to Tick Performance and they say bearings looked ok and everything was fine. Faulty clutch or install? Dealer says clutch looks good.

What can it possibly be at this point? I've covered everything! I'm praying that the rebuilt trans solves it.

FuzzyLog1c

iTrader: (4)

Interference between something and the bottom of the shifter linkage?

LS1Steve

Shifter is a UUC, and linkage is stock. The UUC was ground down to fit the new driveshaft, and create clearance. No evidence on driveshaft of the shifter ever touching it. Plenty of clearance between the exhaust and driveshaft too. Geometry of the new 1 piece driveshaft is different than the replaced stock 2 piece, and this sound was unaffected.

FuzzyLog1c

iTrader: (4)

Odd. Based on your testing, it really seems like the issue is transmission related.

DMM

Does this only occur at speed or will this occur sitting still with accelerator modulation?

LS1Steve

Only occurs when car is moving in gear... cannot hear anything when stopped, no matter what RPM or clutch position. Revs like brand new when stopped and out of gear or in gear with clutch down, or, same when moving, smooth when revving out of gear (clutch in or out) and in gear (clutch down). If I really want to emphasize the noise, i can roll at 2k in first or second, and push the gas and stay on the brake simultaneously for extra load... ugh

Best way to describe the noise is as if the carrier bearing was bad and the DS was hitting the floor or exhaust on every revolution (but seems to be more forward from there)... and again, it is in sync with the flywheel side... when I shift, the noise resets to a slower speed and winds up again when accelerating. It is significantly loud, enough that anyone driving it would be like "wtf is that?!" I'd have bet my left nut it was input shaft bearing. Still hoping its possible to have a bearing that is bad enough to cause noise, but not bad enough to be evident at visual inspection. Transmission going back into car next week, which will be very telling, but just trying to plan ahead so I can help them diagnose it when its still in the shop. Something is definitely not spinning straight.

Onefast V

iTrader: (3)

Is there any issue underneath the car where something should be hitting the frame/body under load and resonating with the engine? maybe your exhaust or your cutouts??

DMM

By default it can only be something from the output shaft of the transmission rearward, likely the driveshaft. If it was the trans causing the vibration only at speed, the output shaft of the would have to be bent causing the output shaft to leak badly (or bad bearings which would be apparent in the fluid).

Check the rear CV of the driveshaft very carefully, the center support bearing is always looked at but the CV can go as well causing this type of vibration if only 1 ball/cage are bad in the entire joint.

LS1Steve

No I've replaced the driveshaft with a gforce and the sound didn't change... new joints on the shaft, no center bearing, and billet couplers, but most importantly IDENTICAL sound as before. Basically eliminates driveshaft. Also the sound frequency would get higher as the car speeds up through gears, which is not my case. This noise increases with engine RPM, not vehicle speed.

LS1Steve

No, no markings at all on the exhaust. I thoroughly inspected it when installing the driveshaft. The new shaft actually increased exhaust clearance throughout (and again, no change with either shaft). Unless maybe one of the fairly new creative steel engine/trans mounts failed? I'll check when its on the rack.

DMM

Is this strictly a noise and not a vibration? Have you tried a chassis ear?

heavymetals

iTrader: (3)

Since it is RPM related do you feel the vibration when you rev it in neutral vs the clutch in?

In neutral you are spinning the transmission (no gears engaged) internals.

Sounds to me like a transmission problem.

LS1Steve

Only felt in gear, at speed, clutch out under load.

completely agree, all signs point to transmission, however Tick stated zero issues found :/

Praying its a bad bearing that just LOOKED like a good bearing under inspection (or hopefully they didn't really even inspect it well, hah)

LS1Steve

Noise and vibration, like someone tapping with a hammer on the floor with every revolution of the engine flywheel

heavymetals

iTrader: (3)

It could be pressure plate related.

Sounds like the acceleration causes enough centrifugal force to cause something to bang around inside the transmission/bell housing.

odthetruth

iTrader: (4)

You sure its every revolution, or is it like 3-4 bangs a second and faster when giving it gas? Probably hearing a bang with every revolution of the driveshaft. (or, I'd bet every 1/3 revolution of driveshaft, considering 3 of 6 bolts stick outward more)

Put all 4 wheels in the air, turn off traction control and stability control, and have a friend give it a little bit of gas and look under. My first guess would be that the propeller bolts are banging into the shifter linkage. I had this exact issue when I had installed my revshift trans mount and the custom shifter I had.

LS1Steve

No, noise was unchanged before and after the driveshaft change. I inspected it well, plenty of clearance with shift linkage and exhaust, and if it was touching the driveshaft, the noise would get faster and faster as the car increased speed... my noise slows on each shift and resets. Noise at 2k rpm in 1st gear is same as noise at 2k in 4th gear... cannot be on back half of transmission... must be front half related.

LS1Steve

Just an update, cannot find any issue w engine or transmission or clutch, so for good measure, i'm going to have the LS7 clutch assembly balanced together before re-installing everything and praying the noise is gone.

raytx1

iTrader: (1)

Although I dont have a cts v, i can kind of relate to your problem. (I have a 98 trans am with the stock 10 bolt)

I have the same knocking noise coming from underneath my car only when i'm in gear and under load. It seems like the noise disappears under high rpms/and heavy acceleration, but that could be because of the exhaust covering up the sound.

I'm hoping its not rear end related, but right now i'm leaning towards gear wear in the differential. Maybe missing a couple of teeth on the ring or pinion gear.

LS1Steve

All my noise is on the input side of the transmission. With a new engine, balanced new clutch, and rebuilt transmission, I'm now left at basically suspecting a crappy OEM balancer. It was replaced recently but they are crap.

Fluidampr will be ordered as soon as its back together and I verify that nothing has changed. ugh