Nils Grimsmo

If I turn up the gain on my R-115SW, then every minute or so it goes "poff", doing a single punch, as if there is a condensator that loads up and suddenly discharges. This happen both with and without music playing, and even when I disconnect the audio input cable (only AC power connected). It is not very loud, but enough to be annoying. Is this expected? Or a common problem? Should I send it to the store for repair?