I had some irregular cracking and hissing sounds on my Harman Kardon HK 3480. Thanks to the tips and ideas on this and other forums I was able to track down the problem to the right channel in the power amplifier. I tried tapping the transistors and capacitors but this didn't enable me to exactly localize the culprit component. Wherever i tapped I got cracking noise. (I even tried adding some vibrations with an electric toothbrush but without avail). Not having a soldering iron with temperature indication I was reluctant to heat the transitors because I didn't want to damage them. Nor did I have any freeze spray around. I had the idea to very slightly bend the transitors (the ones not on a sink) forwar/backward. This solved the problem! Unfortunately I bent all 8 of them at the same time so I wasn't sure which one was the culprit.. I tried putting them back in their original position but this didn't reinstate the cracking noise... So i repaired the receiver without repairing it 
.
Most likely there is an issue with the soldering somewhere but an inspection with a magnifying glas didn't show any damages. I closed the receiver back up and it has been working a week now without issues.
If the problem would re-appear in the future I will simply revisit those 8 transistors and bend them slightly one by one, checking every time if the cracking is gone. If gone, the last one I bent will be the one with problems.
Thank you all for your tips on this forum, they helped me to get ahead in solving this problem. Hopefully, my solution will be able to help somebody in the future.
. Most likely there is an issue with the soldering somewhere but an inspection with a magnifying glas didn't show any damages. I closed the receiver back up and it has been working a week now without issues.
If the problem would re-appear in the future I will simply revisit those 8 transistors and bend them slightly one by one, checking every time if the cracking is gone. If gone, the last one I bent will be the one with problems.
Thank you all for your tips on this forum, they helped me to get ahead in solving this problem. Hopefully, my solution will be able to help somebody in the future.