R107 fuel harmonic

[longbeach]

I have a 1979 450 SL, with 140kmiles. It runs fine aside from an annoying intermittent problem. Every once in a while it will start this fairly loud whooing, almost foghorn like sound. It took a while to narrow it down,because initially it would stop soon after the vehicle stopped. But recently it has changed it's characteristics some, and the other day I was able to search around a bit more. Originally it seemed to be coming from under the air cleaner housing. But now has moved to under mid-body of car. When this happens I can actually feel the fuel supply line in the engine bay,right where it changes from hose to hard line vibrating. By wiggling the line back and forth I can change the sound of the harmonic. Now I'mtrying to figure out if my problem is in the fuel distributor system,or back aft. I've already changed the fuel pump and filter, because I thought it may have been fuel pump related. Any ideas or input greatly appreciated.