When completely stopped with brake pedal depressed (as in at a red light), the engine idle drops to around 500 rpm and the engine runs rough - even, occcasionally, to the point of stall. Does not occur in Park or in Drive with parking brake on, only in Drive with brake pedal depressed. I'm stumped, any clues out there?

There are basically two types of whistle noises.

* Whistle when you get more gas (increase RPM, put gas pedal, sometimes just idling but louder when you put more gas)
This usually intake-side leak.
Intake manifold itself, intake manifold gasket (most likely), even airfilter housing, Airflow meter housing (connector), anything where air goes to engine.
usually engine get rough or unstable idle when intake leak occur.
Engine knocking or pinging may feel.

* Whistle when you decrease RPM (engine brake, reduce speed, sometimes just idling but louder when it dropping RPM)
This usually Vacuum-side leak.
There are many vacuum hoses in engine room. even inside of cabin (heater control, etc)
Try to spot where it coming from.
Vacuum also connected to Brake servo, heater control and spark timing.

Put metal or plastic pipe (small PVC pipe will work fine) to one of your ear then isolate source of noise and find where it come from.

Clean the throttle body inside and the idle air control motor.This will help.

Hi Wendi:
If the engine runs fine except when you don't have your foot on the gas, maybe the idle stop needs to be adjusted. With the engine off, have someone step on the gas pedal while you look at the linkage at the carb. There should be a plate with a screw in it that keeps the butterfly valve from closing completely. This can be adjusted with the engine running. (SAFETY FIRST - MOVING PARTS CAN BITE)
If the engine will only run at not nice too high RPM, chances are that the idle jet in the carb is plugged, and a carb cleaning and possibly overhaul may be needed.

fuel injectors may be clogged and not spraying properly

You have a mechanical bind. Disconnect the main linkage and work it all by hand. Everything should be free and slopy. Aerosol carb cleaner will help, but only when the engine is not running.

I'd say sounds kinda like a missfire although with the absence of an engine code I would lean more towards a vacuum leak somewhere. Chk all your lines while cars running to see if you might have a vacuum leak.

Check the help links related to this problem.Click the link below to go through its troubleshooting procedure :-----Idles rough and quits when gas pedal is pressed less? http://technoanswers.blogspot.in/2012/01/1999-ford-f-150-idles-rough-and-quits.html---------
Car won't start without hitting the gas pedal? http://schematicsdiagram.blogspot.com/2011/12/car-wont-start-without-hitting-gas.html
-------Complete loss of brake pedal then all gets normal? http://repairhelpcenter.blogspot.com/2012/01/complete-loss-of-brake-pedal-then-all.html
------Car dies when gas pedal is pressed? http://schematicsdiagram.blogspot.com/2011/12/car-dies-when-gas-pedal-is-pressed.html
--------Pushing on the gas pedal, but the car slows down? Gas pedal to the floor and the car slows down?

http://technoanswers.blogspot.in/2012/02/pushing-on-gas-pedal-but-car-slows-down.html

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After replacing brake rotors and pads, brake pedal remains on floor? http://technoanswers.blogspot.in/2012/02/after-replacing-brake-rotors-and-pads.html

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accelerator pedal sensor? http://technoanswers.blogspot.in/2012/02/2009-chevy-impala-accelerator-pedal.html

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For rough idle issues and problem troubleshooting,go through the help links provided below :----- van idles rough?

http://howtobyme.blogspot.in/2012/01/2004-ford-freestar-van-idles-rough.html

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will not idle when cold?

http://technoanswers.blogspot.in/2012/02/ford-explorer-40-will-not-idle-when.html

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runs lean and idles rough?

http://technoanswers.blogspot.in/2012/02/ford-runs-lean-and-idles-rough.html

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Engine stumbles coming off idle when warm?

http://repairhelpcenter.blogspot.com/2012/01/engine-stumbles-coming-off-idle-when.html

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Erratic idle on car?

http://schematicsdiagram.blogspot.com/2011/12/erratic-idle-on-car.html

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Low idle when cold out side?

http://repairhelpcenter.blogspot.com/2011/12/low-idle-when-cold-out-side.html

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Idle Air Control (IAC) Valve Replacing?

http://schematicsdiagram.blogspot.com/2011/12/gmc-idle-air-control-iac-valve.html

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Engine quits at idle?

http://repairhelpcenter.blogspot.com/2011/10/engine-quits-at-idle.html

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Car overheats and Air blows hot at idle?

http://howtobyme.blogspot.com/2011/11/car-overheats-and-air-blows-hot-at-idle.html

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While stop engine idles like cylinder missing?

http://schematicsdiagram.blogspot.com/2012/01/while-stop-engine-idles-like-cylinder.html

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This will help.

Thanks.

Inspect brake booster vacuum line from engine intake to brake booster for leaks also inspect brake booster check valve, more than likely your brake booster is causing a vacuum leak to your Intake thus leaning mixture wich increases idle rpm

could be numerous reasons. this is your best bet take it in to a mechanic and have your engine codes scanned. it is a minimal charge, some even do it for nothing it only takes 10 minutes.this will point you in the right direction.

have them check your IAC (idle air control) or just unplug it and see if it runs better. if so then replace it.