Gears & Axles - bought used 10 bolt - whining..

I bought a used 10 bolt recently rebuilt with 4.10s - relatively low miles (7000). The PO said it was in perfect shape. I opened it up and the gear pattern looks perfect and everything seemed to be in perfect condition. I installed it after changing diff fluid. I drove it home and had a considerable whine especially in 6th gear at cruising speed. The PO had been running Royal Purple diff oil and I replaced with Synthetic Valvoline 75w90. I know there is a chance that its just the way the rear was setup but the PO claimed it was perfect shape and seemed to be a reputable seller. I was wondering if changing to a different diff oil would possibly make a difference. I just installed it last night and drove it home and everything seemed to work fine other than the whine. Anybody have ideas on what might cause the problem? I'm going to drive it more and try it out more and see what I can come up with but I was thinking that the different diff oil might cause the problem. I appreciate any help

It would be a good idea to find out whether or not the diff requires the posi additive. Also, being that has aftermerket gears it could be normal. Depending on the brand some aftermarket gears will whine a lot while some will not hardly at all. Get some of this info from your seller and better help can be provided. Also may not hurt to take it to someone local who does rear ends to have backlash etc checked. If you said the gears look ok with nothing abnormal I wouldnt worry too much. For sure check and see it it requires the additive though. Good luck.

Yeah I forgot to mention that is has the torsen diff and does not require LSD additive