Q: Allison trans in 2001 gmc 2500hd will sometimes "whistle" when put in reverse

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When trans is hot and shifted into reverse (when stopped) it sometimes emits a "whistle" sound very much like a "backup warning" on a garbage truck. It is intermittant and does not seem to effect performance. Smooth shifts, trans temps getween 150f and 175f. New filter abd fluid in last 1500 miles

My car has 65000 miles.
My car has an automatic transmission.

It is possible that there could be a restriction in the flow of transmission fluid when in reverse. I would recommend a power transmission flush to resolve that issue. It is also possible that there could be an issue with the transmission pump. I would recommend having your transmission inspected by a qualified professional to determine the exact cause of this issue

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