I was on my way into town last night and started to hearing something a bit raspier, I rolled down my windows and shut the radio off, and sure enough it sounded like i was cruising with no muffler. After pulling off the the side and realizing my muffler was still intact and had very little flow coming out of it, i thought it was a bad leak...well long story short, it was, but through what looks like should be an O2 sensor before the cat converter. Bad thing is that i have never noticed a sensor their since ive owned the car and dont see anything dangling. I had an engine rebuild over the summer and it may have been removed accidently by the shop then, but still, what gives? Why didnt it do this before? Hope this pic helps if it uploaded

 

by the way the only mods i have are K&N filter, flowmaster exhaust and !MAF screen. I know, noob mods, college student with no cash. LOL!

 

Looks like an 02 sensor was there at some point. If it was removed and the threads are still ok put a plug in it.

 

A little more info will help here. Year and engine of the car in question and maybe a better pic with the location of the said whole is on the system.

But, like said above, looks to be a O2 bung whole of some type, can plug it for now, but should figure out what was in there and replace it if its something that monitors the cars performance.

 

Car is a 1999 Firebird V6. The REALLY weird part is that i had O2 codes out the ying yang for a month, then this hole started making more noise, and letting out more air then the codes dissapeared?

 

The threads on the bung look pretty deteriorated. I'd guess it hasn't had anythings fastened in there for awhile. Would this bung be post Cat?

 

No, It's pre-cat, which is confusing to me, it had to have had something in it before or else it wouldnt be making this noise til now