Do as Tom said, power of elimnation, possibly RF is trying to get in, a 20-yr mic hummmm, I would try moving the cable off any metal such as the drop ceiling grid, this can act as an antenna, also try unplugging the XLR connector from the hanging mic (there is an XLR connector on a 25-30') cable attached to the mic. find this connector & unplug, plug back in, do this a few times to clean the connector from any corrosion that may have gotten on the pins, althrough you will not be able to see the corrosion, believe me its there....this too sometimes will make the noise go away, usually pin #1 shield is at fault, sometime works sometime not, just remember any connection in the chain could be at fault, a connection is the weakest link.... IF it is RF....you can install a Shure RF Filter in line with the mic.....if really bad you will have to run a high grade shielded mic cable such as Canare Quad, and/or install a Shure RF Filter.....good luck on your mission.....