Two questions (170 engine)

My bone stock 170 cu. in engine runs great and quiet up to approximately 45mph, and then has a louder resonating sound beyond that speed. By letting off the gas slightly and just feathering the throttle, the noise settles to a more tolerable roar...

Is this a sign of an exhaust restriction (have a new muffler installed) or is this just normal?

Question 2.
A short time after shutting off the (warm) engine, the garage has a strong gasoline smell which goes away after the engine cools (approx 45min-1hr.) Could this be caused by fuel in the carb boiling from the heat of the exhaust manifold sending vapors through the open air filter housing?

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