98 2.4L Taco whines when hot. Can't identify vacuum leak

This is a basic, 2.4L auto Taco with 120k on it. I've had a P0420 code for almost a year now. If I reset it, it comes back on in usually 2-5 days under a variety of circumstances (startup when warm, when cold, on highway, at local road speed, etc)

I have done everything I was told to do in this thread so that's isolated. Nothing here made any difference other than lightening my wallet: https://www.tacomaworld.com/threads/2-4l-reg-cab-tacoma-getting-awful-mpg.359685/

The one remaining symptom that I've never been able to solve is a high pitched whine at or near idle. It happens like this everytime: I start the truck cold with no whine, drive until it's fully warmed up, and then eventually I'll let off the gas (dropping to idle speed) and the whine arrives and doesn't go away unless I'm on the gas. Sometimes it gets louder with just a touch of gas, but always goes away beyond that. It is definitely related to the warmup cycle because I can be stopped while the engine drops RPMs from reaching running temp and with that drop comes the whine.

I'm 100% positive that this noise is not rotational (i.e. coming from a pulley). It sounds like it's coming from the throttle body area. But I have wiggled, pinched, and inspected every hose going to it and nothing has affected the nature of the whine. It's also counter-intuitive that the idle drops with the arrival of the whine, when a vacuum leak usually results in an idle speed increase. In addition, I have sprayed starter fluid in the area of the noise and used smoke to try to identify a leak to no avail.

I don't know what else to do. My next move is to remove the TB and see if anything looks abnormal, and replace gaskets. Before I do this, has anyone experienced this problem? I'm hoping that this will solve my P0420 issue.

 

cleaning the TB and IAC are always good ideas. get new gaskets while you're at it since they will cost under $10.

 

I've ordered new TB and IAC gaskets and am going to clean these things thoroughly this weekend. Through some troubleshooting this weekend I'm leaning more towards the IAC as the source of the whine.

Also, I discovered a crack in my exhaust manifold, so there's the P0420 source probably. Got a new one on order.

 

I have a 1999 Toyota tacoma 2rz-Fe (funny mine is purple as well) and i have the same problem. I was wondering If you found any solutions?

 

Yeah, turns out it's whining because it's unhappy with the color it got painted

Seriously though, I've got the TB and IAC pulled out and have cleaned them both thoroughly. The IAC was a little dirty, but felt like it was moving freely before I cleaned it. The TB was pretty gunked up on the manifold side of the throttle valve. I cleaned it and all it's ports thoroughly. A new theory of mine is that the gunk created a 'whistle' by restricting flow around the small space between the closed throttle valve and the body. Other than the gunk, I haven't seen anything abnormal.

I was sent the wrong gasket for the IAC so I didn't reassemble all this yesterday. I'm picking up the right one after work today and will put it all back together this evening. Will report the results when I'm done.

 

Well, no parts store had the IAC gasket I needed, so I just put some rubber rejuvenator on the old one and reused it. Got everything put back together, fired it up, and the whine is pretty much gone. It's still kinda there but it's waaay quieter. Hard to say what the source was exactly, but the ol' take it apart, clean it, and put it back together trick produced results. New TB to manifold gasket, too, but I didn't see any sign on the gasket of a leak (I would expect to see discoloration on the gasket at wherever air was getting in)

Next stage: replace the exhaust manifold. Just need a weekend day where it doesn't rain in Texas.

Also: I just drove it to work and it definitely responds better with smoother acceleration. Definite performance improvement

 

Thank you so much for the tip ill give it a try myself!

 

An update on this issue: the whine is back. I've put about 6k miles on it since the fix and it's about as loud as it was before. When I pulled the TB off before, I didn't see any signs of leakage on the gasket. I also doubt that the TB has gotten as gunked up as it was in just 6k miles.

 

Wild-ass-guess here, but could it be the A/C compressor cycling? When you have the vent control set to defrost, it uses the compressor to remove moisture (even though the A/C button is off.

 

Thanks for the input, but the issue happens independently of AC operation.