2004 A4 differential shudder/chattering
Hello. I have a 2004 A4 with 27k miles on it. Everything is stock except a LM2 exhaust, Superchips, and wheels. So this spring I got my oil changed at the nearby Valvoline Instant Oil Change. While I was there, the folks at VIC asked if I wanted to have my rear differential serviced; the fluid likely had never been changed by the first owner, and I didn't have it changed during the two years I've owned the car. I said sure go ahead, but it has an LSD and needs additive for that. They said yes we have that available, so I had them change the diff fluid.
After leaving the place, I immediately noted a little "da-da-da-da-da-da-da" feeling coming from the rear end when turning. Not a sound, just a vibration like you're going over a very minor rumble strip on the highway, but coming from the rear end only. I didn't think a whole lot of it, but was aware of it and was hoping it might go away on its own. And coupled with that is backing down the driveway, which is slightly curved there is an occasional light "banging" sound from the rear end too.
So after a couple months of that and nothing changing, I took the car to my mechanic and asked him to inspect the rear end and do a rear diff service on it. I mentioned to him too that it has an LSD and he said yep he understood about the additive. He inspected and drove the car and said he didn't find anything wrong, but he did the rear diff service as I asked. I got the car back from him, and all seemed well for a day, then back to the same vibration and banging from the rear end.
I've convinced myself after doing research on several GTO forums that both VIC and my mechanic probably put in rear diff fluid that may have had an additive in it, but they did not specifically put in a type F friction modifier. I've since then ordered some Torco RGO and Torco Type F friction modifier, so this upcoming weekend I'll change the stuff out myself.
My question is does driving without the right friction modifier in the rear diff screw up and damage the diff? I've put on a few hundred miles of around town driving, little bit of highway since having the diff serviced the first time at VIC, and I'd hate to see the rear end f'd up because of all of this. It wasn't like the diff was running dry or didn't have any additive in it, just apparently not the right additive.