non-RPM sound

so for the last few months, as if I remember correctly, co-inciding around the time of the installation of the SYE.
I have an odd sound coming from under the rig. it doesn't increase with rpm, but does increase with speed.
its almost a faint knocking sound, which turns to vibes at high speeds (well the knockin is so faint that its more a vibe than)
my thoughts were maybe the x-case chain is overly stretched and in need of replacing. but would that cause a sound like this?
other than that, I never noticed it before the sye, and when I pulled my front d-shaft to see if it was ujoints it changed nothing there (rears were brand new)
thinking about it now, I put the rubber sleeve for a shock absorber on the rear d-shaft (a front moved back) just to keep the slip clean... no other purpose. I wonder could that be makin the wierd noise? I'll cut it off an find out, but any other ideas on the off chance this doesn't work.
performance isn't hindered, its only an annoyance more than anything.
thanks

A driveline vibe could cause premature wear of other drivetrain parts usually starting with transfercase items. When was the last time you had you rear shaft balanced?

well I didn't get the rear balanced at all, so that could be it. it just seems tho that at low speeds its a knocking sound. if it were just vibes I could see the balancing being a concern, but with the constant knocking I am looking more towards something loose, stretched, hanging... and I can't see or notice any of those.
The stock weights are on the rear shaft, new joints, and the sye turned true when it was installed and I watched it spin before hookin up the shaft.
either way it maybe time to look into a new t-case. switch the 242 for a 231.