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weird noise AFTER the engine stops

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On the last few events, my '04 makes a weird rumbling noise for about 5 seconds after the engine is killed. These races have been tag-team cross country / woods events of about 35 to 40 minute laps, so the bike is good and hot when you arrive for a rider change.

Kill the engine and it makes a noise like the absolute worst wheel bearing you can imagine, or maybe a tin can with some water and a handfull of rocks being swished around.

Keep sitting on the bike while refueling, and it sounds and feels like it is boiling as well. I have the front frame downtube set up as an overflow catch tank, so it is not possible to see how much coolant is being puked.

Engine is otherwise in good condition, with about 12 hours on a recent piston replacement (13.5:1). Crank etc checked out good at the time.

Have had the top end off again since this noise and overheating started. I suspected a possible head gasket leak causing the boiling, but it all looked good inside. Did the sandpaper on a sheet of glass trick to ensure the head and barrel were flat (which they weren't). Reassembled with new OEM gaskets and correct torque procedure, but it still pukes and makes the rumble noise.

I can't imagine that the engine keeps spinning at all after it is shut off, so I think I can rule out any internal engine parts.

Can the boiling / overheating create the rumbling rocks noises?

Any help or theories appreciated.

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This might sound off the wall, but if the bike is really hot could it be the fuel, I have read about that in the past? You said it feels like boiling when you are doing a refuel. So I would take a look in the tank with a flashlight next time you feel this. Or its just the coolant, do you have a clear line to your catch tank? Just a thought...

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i have also heard of the fuel boiling. Try drilling a very small hole in the cap to help release pressure on long hot rides, and see if that helps at all. Not sure what type of coolant your running but ive run engine ice for years and have never had a problem with it.

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