loud hum/whine from inside my ac/heat ducts

I have something inside my duct work that is vibrating and howling in the wind when I kick the AC on high. At first I thought it was my ac/heat blower motor going south so I changed it out for a new one. I should have known that was not it as the noise stayed constant not depending on speed of the motor, just louder or quieter with more or less air mass moving through the ducts. It does it on MAX, NORM and BI-Level. It does come and go but when I hit the breaks it really comes to life. If I put my hand on the underside of the dash when it's howling the duct work is vibrating like crazy. It's a whine type harmonic vibration not a deep rumble kind of one.

Anyone been inside the AC duct work to see if there is something that could be flapping in the wind at a high resonating speed to create the whining howl?

Thanks in advance. ==== DT

Are you sure it's not leaves or small animals hiding somewhere near the blower motor? I live in the woods, and it is surprising where we find acorns, grass seed, and anything that critters can use for nesting or bedding. There's always acorns in the air cleaner boxes of our XJs, and they chewed through the air induction tubing of our Outback. Years ago, we had a Taurus, and the varmints chewed through the plastic mesh over the cowl, came in through the ventilation, and set up shop in the glove box! Every time you put the fan on, the "squirrel cage' of the blower would vibrate, as it was out of balance, and soon all kinds of crap would blow out the vents.
Take some of the ductwork apart, and you may be shocked by what you find.

Sounds like a vacuum leak to me. check all your lines for cracks and breaks, also check the line running under your battery to the vacuum reservoir in the front bumper on the passenger side.

An easy way to see if the vacuum is working/strong enough: Does the airflow come from the proper places. That is, windshield vents on Defrost, floor on Heat, dash on A/C, dash and floor on Bi-Level. Or is the pitch/frequency of the noise dependent upon the blower speed? When you turn off the blower/HVAC system, does the noise-vibration stop. If the fuctions follow what you select, the vacuum is probably OK. This sounds more like something is physically stuck in or near the blower fan.

The air is directed in all the right places based on the selection on the switch/slider. The noise stops when I turn off the AC/Heat blower, well actually it dies off as the air movement stops. It's in relation to the air movement but not directly proportionate to the speed. meaning if I have it on high or low it's the same noise. If I have it on Max, Norm or Bi-Level same noise. To be honest though I haven't tried it on the defrost setting. I'll give that a whirl tonight on the drive home.

I guess i should have looked inside the air ducts more when I had the blower out. I regret not doing that one. Not that it's brutal to remove and install as I thought it was going to be.

So the mention of a vacuum leak at first I blew off but now that I've really started to monitor everything going on when the hum comes and goes. It happens more times when I let off the gas and hit the breaks in rush hour traffic, almost never on the highway, and as soon as I hit the gas it goes away. So I was thinking it was momentume maybe but then the vacuum leak jumped in my head so while foot on break and it humming along I slide the trans into N and revved the engine. Instantly the hum would go away and when I'd get back down under 1600rpm it'd return loud as ever.

Hmmmm.... I wonder if one of the flapper valves is shuddering under the load with a lost of vacuum?

Could be maybe. Grab a vacuum gauge and start testing.