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Hello: my new DF140 was making an obnoxious squealing noise at 1800 rpm. It sounded like a shaft rotating without lubricant. It had me worried. The marina where I bought it examined it and concluded that it was some kind of resonance effect with the boat - not the motor. It only made noise at 1800 rpm in forward drive. In neutral or reverse, there was no noise. I didn't buy that so I dug further. I removed the motor cover and went for a spin - no noise! I put the cover back on , and at 1800 rpm, with it whistling away, I had my wife hold the wheel while I climbed back and put my hand over the air intake on the cover - no more noise. The air intake has some sharp edges which must act like a whistle at certain rpms and air flow unique to my boat. So if you have an rpm-dependent noise - check the motor cover's air intake. Hope this helps.
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Motor whistle
Interesting - I bought a new 9.9 high thrust and had the same issue. I was told the same thing by the dealer. They changed propellers with different pitches on the 9.9 three times. The last prop seemed to do the job. Still a tiny whistle at a certain RPM but nothing like the howl I got from the first prop.
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