Hi
recently i had my belts replaced and at the same time exhaust leak fixed

now. when i accelerate and i reach ~1850rpm i hear/feel low but loud hum from underneat the car it stops at ~1950, does not matter which gear.
it seems to stop after driving for 40min+ (after something warms up?)
i can't seem to reproduce it when in neutral or parking.
i have AT and 85K miles
what could it be? soemthing messed up with belts? or something with exhaust?
before the repair my exhaust was so loud anyways, that i didn't hear this noise.
is it my muffler? one of the "outputs" is really black.