Microphonic-like squealing from signal chain

[aron]

My friend's system has a tendency to start squealing in a microphonic-like fashion when he uses some overdrive pedals. The signal chain goes like this:

guitar (G&L tele)->overdrive->Digitech floor pedal->volume pedal->amp.

About the only thing I can think of is the Digitech, but why would it start squealing like that? It's happened over the years with him -  with different guitars too. He's always had some sort of Digitech processor, whether it's rack mount or floor-based. It's almost like the signal chain is microphonic. Very strange....

For example, a Dano Fab Overdrive will be fine, but a BSIAB2 will not. Higher gain? Maybe.

Any ideas?

I will start eliminating pieces (like the Digitech) once I have a chance again.

[andrew_k]

For example, a Dano Fab Overdrive will be fine, but a BSIAB2 will not. Higher gain? Maybe.

Output buffer on drive pedal?