I had the wife turn it off and on today so I could try to pinpoint the squeal. That's just about impossible. I think it may be coming from the tensioner as Viper mentioned...at least part of the squeal. I went ahead and ordered a Motorcraft version as the one I have is a cheap aftermarket unit.

I also noticed that on the top pulley (in the middle next to the power steering pump) the belt rides just on the inside lip, not 100% on the flat part. I'm not sure if this could be causing a squeal but I know it's not right. I checked and there is no washer to remove to get it a little closer. Any ideas?

You can move the bearing inside the pully a bit.I swaped bearings in that pully this morning.

Cool. I'll check that when I replace the tensioner next week. Thanks

Got the new Motorcraft tensioner installed and now I'm down to just a little squeak when I start it up. Much less than before. I checked the battery and I think it may be the remaining issue. It was at 12.30 volts and I watched it decrease within 30 seconds to 12.17 volts.

Here are some comparisons pics of the tensioners.

After 1.5 hours it's down to 11.98. New battery time:(. Damn Optimas.

Is the alternator plugged in?

Just installed a new battery. Now the squeak is even less. Not sure what else to try.

Ended up going with an Interstate MEGA-TRON PLUS 78DT.

I had a thought...I wonder if this is no longer the serpentine but my MAF filter sucking in a big gulp right at startup. The sound is more like a short whistle now.

I can hear my 70mm sucking air, much more than my 55mm MAF. It makes a funny noise when I shut my truck off. So yours very well might be making noise. That 427 must use up a lot of O2.

Yep....I can definitely hear mine sucking air. But a whistle makes me think there is a leak somewhere.

JD said his 70mm MAF whistled at idle on his 351.

I chased around a belt squeak / chirping for a long time. It was the top idler pulley. I tried replacing the bearings but I couldn't get them to seat completely flat/perfect. I eventually dropped the $25 for a new complete pulley and that fixed it. However, I understand the squeak too. It's friggin annoying. I could barely tolerate being next to Rox's rig when he started it.

My 400 makes a sort of supercharge whine moan through the air intake.. maybe take the filter off and see if it goes away?

your hearing the idle air controller open up when cold. You wanna hear a loud one my car is loud.

Have some one start it when its cold(24hrs) and have your head under hood. Listen right around the air filter. Bet thats what your hearing. Another thing to try and see if thats it start it and give it just a small rev. Once the throttle opens it should go away.

At least one other guy on CB has a persistant startup squeal and he has replaced everything, except the Driven Auto crank pulley.

I pulled the air filter off...no change...but today it sounds more like a squeal again. I sent a message to Driven Auto Parts to see if they'd be willing to ship me another crank pulley to test with.

Finally got around to purchasing the new Explorer balancer/pulley assembly (28OZ) from Damper Dudes. While it wasn't cheap, and removing installing it was a bit of a process (drain coolant, remove radiator, remove belt, remove fan/clutch assembly, remove/install balancer, refill coolant etc.), I HAVE NO MORE STARTUP SQUEAL!

I believe the Driven Auto pulley only works (without squeal) on 289/302/5.0L Explorer Front Dress installs. It will function on a 351W block...but may squeal.

I think they don't cut the grooves correctly. I've seen some close ups of both and they are different. The squeel is more noticeable with a higher amp alternator.

Happy to find this thread still alive. The 427 is still purring away great. Will try to make the breakfast tomorrow.