I think the first response is correct. The same thing is true with my M3A, vintage 1998. Dust and dirt get into the attenuators producing noise when changing them. It has no effect on the performance. AI offers two deals: upgrade depending on year costs up to 850. Trading yours for a new one with AI costs 1390. IMHO, a little noise when changing the attenuator isn't worth either price presuming that the preamp is functioning correctly.
Audible Illusions Modulus 3A problems
I own an Audible Illusions Modulus 3A and it has the stepped attenuators. the right channel has started making very loud popping noises when changing settings. Once it is in a set static position it sounds fine. I used to be able to spray it with a cleaner and it would operate silently for a month or so but now it pops no matter what! I'm reluctant to send it back because I've heard horror stories about getting them back in ones own life time from the factory. Any suggestions ? Is this common? Any fixes? Is the factory the ONLY choice?