I'm also having trouble with the trem/vib, when i turn on the effect there is alot of noise behind it like an ocillating snowy tv sound.
Hiss and noise can be caused by a lot of things, but if the noise pulses with the speed control, that at least leads you to the front part of the circuit for the source of the noise.
It probably is a bad transistor, but it could be a resistor or a cap going bad too.