Humming Noise When at Part Throttle - Ford Powerstroke Diesel Forum
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Old 08-16-2011, 11:32 AM
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Humming Noise When at Part Throttle

I just bought this truck used and in the first 500 miles I noticed that while cruising at 70mph indicated (76mph actual) and on cruise control, when going downhill and the engine was just coasting/idling I noticed a fairly sharp rattle or hum that I could feel in my butt and my feet. I could replicate it out of cruise control but the hum wouldn't be as sharp or "rattly".

My first though was the truck had a Baby Jake Brake, second thought was a heat shield from the exhaust system rattling and vibrating. Now I wonder if is merely the front bearings as I noticed my dust seal in shot on the drivers side.

It all seemed to quiet down in the next 2000 miles but is still there, only very dull now.

I know it's a long shot, but any guesses for a newbie?
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Old 08-23-2011, 09:02 AM
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If it has aftermarket exhaust it could be the larger downpipe vibrating slightly against the firewall. I've had that problem on two different trucks, and it was only every now and again at part throttle.


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Old 08-23-2011, 10:25 AM
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Best I can tell it has a 4 inch aftermarket exhaust, the muffler is Flow Master and the CAT is very large, I assume a high flow model, and I would have to assume the down-pipe is aftermarket, it is about as large as the rest of the exhaust.

Thanks I never would have considered it.

I did the shake test on my front CV's and they are both loose...but down-pipe makes more sense as it happens during throttle variations, not necessarily speed variations.

Is there an easy way to identify a stock down-pipe versus an aftermarket?
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Old 08-24-2011, 03:32 AM
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Mainly size. Stock down pipe is 3", aftermarkets are typically 3 1/2" into 4", and are typically two pieces clamped together, to ease installation, however there may be some one piece aftermarket downpipes out there.

Hope this helps you!


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