Roaring noise when ignition is turned on

I have a 1978 Buick Lebre custom that is in excellent condition. However it has been stored and not started for about 4 1/2 years. I have changed the oil, put in a new radiator, fluched the transmission, put on new tires and the car runs well. However, when the ignition is turned on a roaring noise comes from under the dash on the passenger side. The noise continuse after starting the car and usually lasts for about 20 - 25 minutes. The noise will return on occasion, but it is not as loud. Anybody out there have an idea as to what it could be? Thanks Frank

Sounds like it could be your blower motor. See if you can disconnect it(hopefully from the firewall) then listen again. If it's still there then check to see if you have a fan clutch. When they start to sieze it can sound like the car is going to lift off!!!That noise usualy gets worse when you rev the engine a little.

Sorry! Forgot to mention that the fan clutch I'm talking about is bolted to the water pump

The speedo might be a problem. The cable that comes of the tranny goes to the speedo under the dash, connects to the speedo, and can give off a loud whirring sound. This isn't the case, however, if the speedometer is functioning correctly.

Sorry ,I did'nt read your post correctly.
If it makes the noise with the key on/engine off then I would go for unplugging the heater motor. Many don't have a real "OFF" position on the speed switch so it will spin even when blower is turned off. There is a reason the manufacturers do this but I forget why.Maybe to help avoid siezing.

Nah, they just want to be sure to find some way of driving you nuts so you bring it in to get fixed when there's nothing wrong with it :) .

The reason I had posted that the speedometer cable might be loose under the dash is that it happened to our '89 Astro, and on cold mornings for about 30 min it would make this aweful whirring noise that was very, very loud. It turnedout that the speedo cable was loose, or broken, I can't remember which, but it was a pain to fix 'cause we had to pull the dash apart.

The seized fan clutch, unlubricated speedometer cable, and worn-out blower fan are good bets, but if the car was stored for awhile I have another idea..........maybe it's a mouse nest in the blower fan area. Happens a lot around here. Sometimes it's easy to check - turn the fan on high and if bits of fluffy stuff are starting to come out the vents, it might be it. This older car may have a easy to remove fan housing on the firewall? Wouldn't hurt to tilt it down and look inside?
Good luck,
Highland