Oil pick-up unit perhaps clogged, or not sending properly? Just a suggestion.
I do not think these models come with an oil pressure gauge.
With one of those, it is easy to tell if you are having pressure drops.
Hopefully it is just something minor.

Or...perhaps it is exhaust related. eg: once it warms up, and the exhaust pipe expands, the noise goes away.

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CHILCUTT~
The secret to a long life. Is knowing when it is time to leave.