HAVE A WHISTLE/WHINE WHEN SLOWING DOWN IN THE ENGINE COMPARTMENT

[DENNYSCARS]

NOTICED A WHISTLE OR A WHINE DOWN LIKE A JET MOTOR (NOT AS LOUD OF COURSE ).  THOUGHT IT MIGHT BE A VACUUM LEAK BUT NOT SURE.  SEEMS TO BE WHEN WARM AND ONLY NOTICE WHEN GOING LOW SPEED AND LET OF THE THROTTLE.  ANY IDEAS/SUGGESTIONS??

[Paul G]

Power brake vacuum booster maybe? You can hear a whistle in the passenger compartment if it has a vacuum leak. When you hear the whistle try depressing the brake pedal and see if it makes a change.

[Purple440]

I had a whistle/whine like that when I let off the throttle at highway speed.  In my case it was a vacuum leak at the intake/carb...I forgot to put a gasket between the spacer and the carb.