Whistling sound when hot

For a few years now, I've been hearing a tonal, airy whistle when the engine is hot and the RPMs are coming down. Typically, I'll be on the highway and come to a stop where the wind noise dies down. The whistle will be high pitch and then drop an octave and then another as the engine RPMs fall towards idle.

I can't locate the sound in the engine bay, but can hear it in the cabin. Vacuum leak? I've tried a propane torch leak check on the obvious vacuum hoses in the engine with no luck.

The first thing I would suspect is the Throttle Body. The Idle Air Controller port has sharp edges that could be the source of the whistle. Next time you have the TB off for cleaning or other work, put a small radius on the IAC port with a file or sharp knife.

Any idea why that'd be heat dependent?

Can try pulling the dipstick out some. I had an old 5.0 that I forgot to add a crankcase air inlet on. Itd pull a lot of vacuum hot and make a whistle like that. If it fixes it your ccv system is plugging up.

Intake manifold gasket.

After some years, the paper portion of the gasket wears thin, and, when intake air
rushes across the edge of the gasket, the metal backing acts like the reed on a
saxophone.

Get a stethoscope and probe around where the intake manifold meets the head while
you blip and feather the throttle and listen where the whistle is loudest.

You can also get a water spray bottle, and set it on "squirt". While the engine is
whistling, shoot a thin stream of water where the intake manifold meets the head.
When the whistle disappears - then you've found the bad spot on the gasket.

Will do. If I find the spot, best to replace the gasket entirely?

The throttle body gasket gets brittle and cracks. Then when you decelerate the throttle body closes, and the vacuum load on the engine jumps way up. Air rushing through the cracks whistles. Easy to replace the stock gasket.

There is another gasket around the idle air controller that is not available from anywhere. You have to use RTV for that.

Recently cleaned the throttle body and the gasket seemed fine. (Live in a dry climate). IAC gasket I need to double check, though don't recall it looking worn.