2003 F150 4.6L Supercrew:

I'm having a relatively new problem with my truck, and I don't know if it is related to the recent brake work I did or not. But I first noticed it after replacing the front brake pads and bleeding the brakes. That's when I started hearing a whistle or squeal once in a while. It occurs for about one second, it seems to happen on a random basis, and it does not sound like metal rubbing -- it actually sounds like a whistle.

This sound is NOT the brake pads or disks squeaking/squealing, so if it is related to the braking system I'm thinking that it may be coming from the master cylinder since that's the general area the sound is coming from. It seems completely unrelated to speed, RPM, acceleration/deceleration, turning, braking, and all the other "common" things you might think of when trying to relate a new sound to a part of the truck.

This whistle sometimes happens even after the truck is parked and the engine turned off! A few days ago after driving around for a while the whistle occurred after I had parked, shut the truck off, and gotten out of it. It happened just before I shut the door to go into the house. It sounds like it's coming from inside the engine compartment on the left side, in front of the driver, and it is a rather loud whistle -- but each time it happens it only lasts for about a second, which leaves me no time to see where it's coming from.

Since this whistle occurred after the truck was parked and the engine turned off, I'm wondering if something in the braking system is being over-pressured? Is there a pressure relief valve on the master cylinder (or somewhere else in the braking system, or somewhere else in the engine compartment) that might be whistling when it releases excess pressure? If so, this might explain how the sound can occur even when the truck is neither running nor moving.

Might this whistle be caused by a vacuum problem instead of a problem in the braking system? The brakes seem to be working fine. The only reason I mention the brakes is because that's the last thing I worked on before I noticed this new whistling problem.

If you have any clues about what this sound might be, and how to diagnose it, please tell me. Otherwise I might start to believe that my truck has become haunted. It is getting close to Halloween after all ...

By the way, I have recently experienced a couple new issues that may or may not be related to this whistling sound, so if you think that one or more of these issues are related please tell me:

1- Just this morning the truck stalled after starting. It has never stalled before. When I tried starting it again, without putting my foot on the accelerator pedal, the RPM's went up to 1500 or 2000 then down until it almost stalled at maybe 300 RPM, then up again, then down again ... as if the computer was unable to find the proper idle RPM. Eventually it stalled again, and it stalled another couple of times before I eventually got it running long enough to drive it up and down the street with some hard acceleration. After that short drive I parked it in the driveway, and the idle RPMs were constant and did not fluctuate in the least.

2- Yesterday, for the first time this season, I turned the temperature control from full cold to full heat, and I switched from the "max air" position to "panel+floor". After the truck warmed up it started blowing warm then hot air as expected. But then I was surprised to note that it decided to start blowing cold air. I did nothing to cause this. Does the vacuum system control the heat/cool mechanism?

I don't know what's up with this truck lately, but the whistling and these other two problems make me think it could be related to the vacuum system. Am I wrong?

 

Yep- sounds like a vacuum leak- easy to say, could be difficult to locate- there is a notorious little elbow on the back of the intake that is prone to deteriorate- i dont have pics, but its relatively to locate- same with the hoses, you might have to just listen and feel around- there are some test methods( smoke test) but i have never done one myself- you tube may be a place to look for instructional/help videos!

 

Check the brake booster... sound like it could be coming from there and if the leak is big enough it could be causing all of your problems.

 

Also make sure there's no residue around your radiator cap on your reservoir, and coolant level is stable. It will squeal in short burst if relieves. Especially after turning off when it goes into heat soak.

But without misfires code, and RPM fluctuation yours does sound like a vacuum issue to me

 

Thanks to both of you for your suggestions!

Apparently everyone else who has read this thread thinks it's a vacuum leak too, or else they might have posted their own comments. I will investigate using the smoke method to see if I can find the leak or leaks.