Hey all,

Weird sounds coming from my 1990 B250, 318 with I believe an A500, and not sure where to look or what to diagnose. I'm pretty mechanically inclined and am at a loss of what to do next, looking for some expertise.
To make a short story long:
I did my intake manifold gaskets cause they're leaking bad, and did my timing chain and water pump. Upon starting up again I noticed a low growling sound when at low speed (sub 20km/hr) and no throttle, also occurring when idling in drive with brakes on. Growling / vibration much more prominent when cold. Once it warms up its damn near gone. Van runs great, can't see it being anything in the motor.
Went to a mechanic he said he doesn't think its a problem, took it to a tranny shop for a second opinion, he said tranny shifts good runs good and fluid is in good shape, didn't think its a tranny problem - listened to the oil pan and **** was wild in there so he figured oil pickup could be plugged/pumps F'ed up a bit. Did my oil pump and cleaned my strainer which was dirty af. No dice.
I think it's gotta be in the tranny, makes noise in D and R when cold and when idling in D with brakes on it'll be like it stalls a bit then RPM comes back tiny little stall then back - not consistently or in a pattern just kinda dies a little bit and fires itself back up. Has no issues idling in neutral. In Drive; give it some throttle and it runs fun, noise goes away, high speed no problems. Something up with Low gear/Reverse?
I'll note that the growling did exist to some extent prior to tuning up, but now its cranked up a notch. Also I've done the U joints.

I'm spent doing work on this thing, but am leaving on a long journey in a weeks time. My next move is throw in a tranny filter and gasket, maybe adjust a band - never adjusted bands before - after that I don't know what I can do with the thing.
Looking for the forums' infinite wisdom. Thanks.