On an evening I'd travel on my local Mway using the old GS1000 to see my girlfriend at the time (1978 model, the bike not the girlfriend). I thought for months they had done some work on the road surface as I got a very bassy loud "bumm - bumm bumm - bumm" as I went along. Then I noticed that I only got this sound between 110mph and 114mph, once I got above this speed it stopped. I turned out it wasn't the road surface, it was the front forks flexing.
I tried to keep my speed over 115mph after this for obvious reasons.