A common recco for sustained cruising RPM with aluminum pistons is 2,500 feet per minute piston speed, which occurs in the 235 & 261 at 3,810 RPM.
The math: RPM @ 2,500 = 15,000 / stroke
The noise can be safely ignored, but if the engine feels "rough" (buzzy through the pedals, steering wheel etc.) it may be a bent fan blade, harmonic damper on the way out, etc. If it's still there when you depress the clutch it's in the driveline, not the engine (bent driveshaft, bad U-joint, wheel balance). Check it before it goes south.
