shhhhhhhhhh noise after about 15 minutes of driving and transmission issues

What is your question?

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!