Almarro has poping sound, please help


Recently, my Almarro 318B develop a loud "poop" sound. It is not very frequent. It is intermittent, usually have it after 15 mins or so after it is started. What should I do? I bought the unit used, and I have no history of the unit.
Should I take it to service, or should I try replacing the tube? Is there a way to find out if a tube gone bad? If I am to try replacing the tube, which one should I try first?
gte357s
Is the sound coming out of both channels or just one side?
If it is just one side, try swapping the left and right output tubes and see if it goes to the other channel. If so, it is a bad output tube. If you don't know how many hours are on the output tubes, the smart bet is to replace them to be safe.
I've had the problem in the past and it was the output tubes. It's time to change the 6c33c's out.
I had a speaker that had a pooping sound one day. Very concerned, I removed its grill to make sure everything was OK. Sure enough, it had had, well, "an accident" with its woofer shall we say.