Ive got a 91 7.3 idi with a banks sidewinder kit 185k. Ive noticed when driving at 45 - 65 mph for a period of time, when i let off the throttle their is a chattering sound and the engine becomes louder until i reapply throttle. Also at any speed when the brakes are applied fairly hard the engine become louder also with slight chattering. I didn't know if this could be some kind of vaccum problem, or timing problem, i don't know. Any help or ideas would be greatly appreciated.

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Also the truck is an automatic 4x4.

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I retarded the timing this evening and turned ip up 1 flat, that fixed my chattering noise at highway speeds but still have louder engine noise while braking. Sounds like brake booster is putting a load on motor. If anybody knows what this would be i would like to know.