What's this noise?

[GEMeeks]

I just noticed a special situation noise in my '97 B3500. When accelerating heavily with a load, rpm above 2000, and release the accelerator quickly, it makes a whiffling sound. Maybe three rattles like something loose in the exhaust system or an air noise of some kind. Just happened to have to jerk my foot off the throttle while taking off from a stop light with my 5th wheel in tow. Drove 650 miles on the way home from Colorado without ever hearing the noise but can produce it even empty by pulling lots of boost then jerking foot off the gas.



Any ideas? Thanks. G. Meeks



'97 B3500, Auto, 4. 10, TST plate (230 hp version), K&N filter

 

[Bob Wagner]

sounds like...

its the sudden shutdown of the turbo



Not available either at high boost

 

[HoleshotHolset]

Bob is right... it's the sudden rush of the air that is sortof left over when you let off the throttle after being into heavy boost. The noise coming out of the exhaust is the wastegate..... you'll hear it BIG TIME if you have no exhuast on your truck like I did for about 3-months or so. You'll hear both if you're in boost and then suddenly you are no longer in boost..... it's normal and won't cause too many problems provided you don't do it all the time... . it is kinda hard on the bearings inside the turbo... but the stock and most aftermarket turbos are pretty well suited to take the abuse. Turboed gassers use a blow-off valve (some even have 'em from the OEM... most are $HP$ add-ons) to relieve the turbo of this stress and to help keep the turbo semi-spooled between shifts.....

Hope this helps,

Matt

 

[GEMeeks]

Thanks, guys, for the info. Don't know how I have managed to miss this sound in 50,000 miles but I think you are right about the turbo doing it's thing. I generally let off the throttle more gently. Believe someone started stopping in front of me and I went to the brakes quickly. I think when I tested it later and heard it again, I pulled boost over 20 lbs then snapped the throttle shut. I guess it is possible that the wastegate might be getting more noisy after 4 years.