I have had several Airhead GSPDs, and they all exhibit a low-frequency 'rumble' from the front fairing, under high-torque, low-rpm situations. It is clearly a rattle or vibration from something loose in the fairing assembly. I've chased this noise for years on several GSPDs, AND FINALLY FOUND IT! The problem is in the 6mm screw that fastens the turnsignal mounting bracket to the fairing frame. There is one 6mm allen screw that fastens the turnsignal stalk to the fitting, and then ANOTHER 6mm allen just below and outboard of the first one. That second, lower allen screw has a big fender washer that is supposed to compress a rubber washer/bushing, and hold the turnsignal mounting bracket tight, but it has a 6mm metal bushing in it that is a couple of millimeteres too long, and so the screw/washer bottom out on the metal bushing and do not apply enough pressure to the rubber washer/bushing, and so the turnsignal mounting bracket is always loose, moves a little, and VIBRATES under hi-torque, low-rev settings. The FIX: remove the lower, outboard 6mm allen screw and washer (it is surprisingly easy to remove - you will need a ball-end allen wrench and vice-grips), and superglue a regular hose-washer to the stock fender washer on the 6mm screw. Reinstall the screw/washers witht the rubber hose washer away from the allen-head. Tighten. The rubber hose washer will now apply enough pressure on the turnsignal bracket to make it tight. No more movement, no more rumble from the fairing.