REL Strata III - faint crackle/pop sounds

I've been very happy with my REL Strata III for many years. However, I recently started hearing a faint crackle/pop sound coming from it whenever it is powered on.

Any ideas for what I can try to do to diagnose the problem? Any recommended repair places? I can try Sumiko (USA distributor) but I would prefer a recommendation from folks on this forum before I send it to them.

Thanks in advance!