Ross Compressor breathing/popping ?

[SuperGeo]

Hello

I finished build an Ross Compressor yesterday. It seemed to work very well but when I stopped playing and then hit an string, a sound like a pop appeared.

I've read an topic that said that the trimpot had to be set in the middle, but doing this don't take the pop away.

I was using two 2n3904 and three bc550 so I thought that it could be the problem so I took off those 550 and put three 2n3904 so all transistors was the same model

but when testing again now the pop still remains

I'm using an 2k2 trimpot, because there was no 2k trimpot here, can it be the problem? should I just put two 1k resistors in place of it?

Thankss