I've recently bought a GK 1001RB-II head and paired it with an 8ohm Laney NX410. Just playing it last night I noticed one and only one of the woofers on the cab seems to pulse a lot even after I mute the string. It's quite low but I can hear it flap. Wondering if it's a fault anyone has experienced before and if they could clue me up on what it could be and how to fix it. Thanks for any advice given.