Receiver makes *Thump* when no sound

[Wojtek]

I've got this problem with my Harman Kardon AVR340 receiver. Up until now I was
connected with analog RCA cables, but today I finally got around to
trying the digital coax.



The problem was also present with the RCA cables, but it didn't bother me much.
A loud "thump" was heard everytime through my left speaker (Klipsch RF-35) when I
changed surround mode, turned of the receiver or pressed the mute button. Quite tolerable and I
didn't make much of it back then.



With the digital connection, however, it's become more troublesome.

Problem: Besides hearing the thump when turning the receiver off,
changing the surround mode, or muting it, it now also makes that sound whenever there
is no audio input.



Ex1: When I pause my PVR, I hear a *thump*, sound plays ok when resuming.



Ex2: When audio format seems to change (between movie announcement 2.0 and intro credits 5.1) I hear a *thump*



Ex3: Browsing some folders on the computer makes: *click sound*,
*thump*, *click sound*, *thump*,*click sound*, *thump*

Ex4: After pausing Winamp, *thump*

I've isolated the problem and it is coming from the receiver. If I swap
the left/right connectors from the back, the thump is alternating from
left/right speakers.



My subwoofer is not affected by the thumping problem, only the left front output.



What could be wrong?