My 2000Avalon has a high pitched whistle. Sounds like it is coming from somewhere around the air intake?????

SOURCE: High Pitch whine during acceleration

First you need to isolate the noise. You don't mention the mileage on the car. Do you have the supercharged engine? If you do, remove the belt for the super and make sure that is not the problem. If you still hear the noise approximately below or from the super area, it is most likely the belt tensioner pulley.
In my car there was a large washer over the pulley (original equipment), as the pulley wobbled on the bad bearings, it would cause a sound like a bad alternator or gear whine getting exponentially worse the faster the engine reved. Now that the part is replaced, you can barely hear the engine idle and is significantly quieter at high RPM.
My car - 2004 Pontiac Grand Prix GTP with 66,000miles.

SOURCE: high pitch whistling noise in engine compartment of 2005 envoy

There may be a small hole or cut in a vacum line or possibly just loose. Also make sure the air filter box and the big hose to it are secure and clamps etc are tight. Pulleys and belts can make a high pitch sound also.

SOURCE: whistling noise in engine and rough idle

check all vacuum hoses,sounds like a leak, try spraying carb cleaner near noise,if engine rpms go up you found the leak

SOURCE: when i give my car gas a whistle comes from the engine. It's a high pitch continual sound. When i let off the gas the whistle stops. It sounds like it's coming from the top of the block.70k miles Thun

Sounds like a vacume leak, chk. for a cracked or broken vacume line or maybe one came off. Does it idle ruff? With it running open hood and listen for a low pitch whistle you may have to rev it up a little.

SOURCE: My 2003 Ford Explorer has a high pitched whistle

you may have a tensioner or idler pulley starting to go bad. They make a high pitched whistle when the bearings are on thier way.

Simply have someone slowly bring it up to 2000 rpm and listen with the end of a wooden broom handle touching the center bolt on each. Be careful not to get caught.

If you're not used to this you may be able to hear it whithout use of the "hearing aid"

Your drive belt may simply need dressing. You can buy this at almost any auto parts store for about 4 bucks. (dont use anthing else as you will cause the belt to stretch creating more issues)