tell me you are pulling our leg. The urban dictionary has a few meanings for this however your sound is the furthest thing from a frap sound! Check it out. I am going to edit a little but your free to check out all the definitions if you want a good laugh...
1)the noise a woman makes while masturbating. This sound does not come from a womans lips. Not the upper ones anyway.
Ex: Hey, whats that noise?
Ah, its okay. Its just mah mom frapping again.
2)a loud fart.
Ex

op went frap, and ruined the spaghetti.
3)a tiny bowel movement that happens accidentally when you're passing gas. The most common cause of skid marks. Roughly the anal equivalent of "vurp," the tiny vomit that accidentally comes up in the back of your throat when you burp. Word derived from the combination of "fart" and "crap."
ex:you'd better check your shorts 'cause i think that last one was a frap.
So the frapping sounds like a wet fart or a women taking special care of her self. There are 36 possible meanings none of which my wing has ever come close to making. A rattle, a thunk, a knock, pinging, bluburing, a whine, a squeak, a creak, a clang but a frap?
Is it posible to capture that noise on your camera and share it! Two members took a stab at the exhaust which can be a source of interesting sounds. Lets hear it and we all can vote on the sound bit and pehaps rename the sound with a more appropriate mechanical tune to it.
If indeed it sounds like a frap i will submit it to the urban dictionary for the 37th definition.
Frapping: "the sound a goldwing makes just before it takes a crap!"
i am having to much fun with this, but really, if you can record it so we all can attempt to pin it down. Thanks for giving us a shot and a good laugh. I sure hope i am not the only one laughing...:rofl: