Sounds like something slapping around in the crankcase; My sister-in-law's car sounded like that, and it was because the timing-belt was shredded and slapping against the cover. I have no clue what it could be on the strom.
The fact that it comes on strong at lower rpms, then begins to disappear at higher revs sounds like possibly sloppy crank or connecting rod bearings, since the motor is not being loaded. Just a random opinion. It doesn't sound good to me.
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