Thumping is 100% caused by the 22UF 50v cap directly between the IRS IC and the Output MOSFETS. If you are hearing that thump, the IRS IC is fine. At this point in time, the amount of time the, 'now brown' glue on these amps is turning very acidic when it gets warm. It etchs the copper, and many resistors, transistors, and diodes fall off the boards when the glue is removed. it's a 50/50 shot at repairing those early versions now. We probably have a box of 30 at this point, we threw out just as many back in October.