I have a Hartke 5000 that I run through an Avatar stack (2x10 and 1x15). On occasion the 15 makes the dreaded farting noise. I took off the front grill and did the basic push test and the speaker seems fine (no scratching). I also took off the back plate and trimmed a little extra solder off one of the connections and at our last gig, I did not have any problems. BUt I'm not convinced this was the problem. I've switched speaker patch cords, switched amp heads and switched cabinets. The only thing I'm sure of is that occasionally the Avatar 1x15 makes the farting sounds and I can see that the speaker is really stressing it's excursion. I've also checked for balance/tightness of the screws holding down the speaker. Since it isn't a constant problem, I'm wondering if someone can suggest some other tests. ATM, I am thinking that the fault lies in the amp. BTW, I do run it in Biamp mode and turning down the bass also seems to abate the problem, but not necessarily solve it. The Avatar is an older one, so it is rear ported and perhaps I've just been too close to a wall when it's happened, but I don't think so. I'm open to any suggestions others may have as to likely causes and cures.