Truck Engine Noise - Increases dramatically above 1500 rpms, then disappears


#1

Vitals - 2004 Ford 350 Diesel 6.0L, Approx 80k miles
Purchased a year ago, driven approx 8k over the year, not a daily driver but typically taken on at least two 45 mile drives (highway, country, and urban roads) once a week but often more and also used for long trips. Truck has had regular maintenance and no issues.

Symptoms:
After loading mulch (by hand so it wasn’t much weight), truck drove fine for 25 miles. After restart (got coffee and donuts), truck was fine through town but accelerating from 25mph to 55mph engine noise was louder and sharper. noise persisted when engine was above approx 1500rpms. After a period of time driving over 65mph, noise stopped and engine behaved normally. Even with the noise, engine never seemed to lose power.

Since that time(one week), about 50% of starts have resulted in the noise with no clear pattern of hot vs cold starts. Even after starts where the noise occurs, if the engine is pushed to 70mph sometimes the noise stops and it not present for the rest of the drive even at those lower rpms where the noise was previously present. I don’t know if the noise would happen in park with a revved engine without going into drive but if I shift to neutral while the noise is occurring, it persists. Noise is definitely not related to vehicle speed but rather engine speed because going downhill if gas is taken off, the noise is not present regardless of speed.

Hope someone has some helpful thoughts or advice before I take it in to the shop.

Thanks,
Steve


#2

If you have a hydraulic clutch fan… check it with the engine off. It should have a slight drag as you turn it. A lot of these fans will stick on and the noise is atrocious until it releases. It’s a very common problem with motorhomes. I always replace them with flex fans as they are quieter and move a lot more air through the radiator.