Pop/crackle in one speaker at only low frequencies in the tweeter

Odd problem here. I have some Silverline Audio SR 16 speakers which I just purchased used. They sound great. However I heard a pop (like what vinyl can do) when playing bass heavy music (not vinyl, digital served). I thought the subwoofer was rattling a connection as it did not sound like a speaker issue to me. I removed the sub and I thought it went away. Today I was running the stereophile test tones to dial the sub in at low frequencies. A pop or crackles appears in one tweeter at 50hz test tones and the few test tones below that. Nothing so far anywhere else in the frequency range but I have not been doing that much. I get the same pop/crackles thing with the sub removed and I play the tones thru just the monitors. It happens about 1/2 the time I run the test tones and only in the low bass area.  I did swap the speakers from right to left and the issue follow the speakers. I am running the speakers single wire and they have jumpers installed. 

Any ideas here?