shhhhhhhhhh noise after about 15 minutes of driving and transmission issues on 1996 Volvo 960

OK, so I bought my car about 5-6 months ago. Within 1 day had a problem with the transmission - felt like it was slipping, sometimes just staying in low gear. Had a rebuild trans put in. Fine for a little while, than there was a shhhhhh sound after driving about 15 minutes, which would stop if I turned the car off and then back on a few minutes later. Mechanic told me it was probably the computer for the trans. Found another one of those and all seemed fine for a about a month. Shhhhh noise is back and so is the transmission slipping/low gear issue. Have a feeling my mechanic doesn't know how to fix/diagnose this issue. Could there be another electrical issue sending codes to the computer? Incidentally, the previous owner had the transmission replaced with a rebuild also before I bought it I've now been told. Took it to a trans shop and he said it had rust in it from sitting out in the parts yard. I will be taking it to a Volvo shop in the next few days, but would like some advice so I know they are being straight with me. Many thanks!

Asked by for the 1996 Volvo 960
I know I'm jumping in here late, but just a thought; does the car seem to lose some power during acceleration. Possible convertor or muffler restriction causing blow back pressure from exhaust. Just a thought.
It loses a little power I think, but not significantly. I'll be taking it to another mechanic in the next few days, so I appreciate everyone's help on this forum.
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2 more answers , 2 more comments
1st mistake is you bought a volvo , second mistake is I would of brought the car back if the tranne went after 1 day ,now watch how much
Dealer bangs you over the head for that ,,,minimum 1000$ ...this car is Costing you a fortune..
Its that pipe under or behind the throttle body ,its a known problem on those..
i think its your blow off valve?
if this car equip with turbocharged
it's not turbo charged...and my apologies, I forgot to mention that it makes the shhhh noise as I am accelerating, but it stops when I get to a constant speed.
Vacuum leak?