Hi

First at all I have to say that my English is little bit rusty, but
hopefully you understand what I try to say.

I have this Sony 7.1 channel receiver which makes a crackle(is it
right term?) noise in front left speaker (A+B) and headphones(left ch.
also). The noise appears when I adjust master volume, input selector
or mute. There isn't any differences if I adjust these from remote or
front panel. When I adjust master volume, noise appears only between
these dB levels, which is strange:

-53,5 -54,0
-61,5 -62,0
-69,5 -70,0
-77,5 -78,0
-85,5 -86,0
-93,5 -94,0

Without this noise, receiver works properly.

I have service manual for this and I already checked these:

-muting transistors - ok
-op-amps and capacitors near audio selector and electrical volume -
ok

What next?