Trumpet after warmup (help!)

Hey everyone, just bought back my first car. It’s a 2004 mustang 3.8 5speed. The owner I sold it to just rebuilt the transmission back in June and just swapped in a new motor w 80k miles last week. The car has a few strange issues though, the first was a random misfire on startup (first 1000 revolutions) and a misfire in cylinders 1,2,4. These were pending codes that went away after I fixed a vacuum leak coming from the upper intake plenum going to the cold air intake. Now, these codes are gone... however, after about 2 mins on a cold start there is a trumpet like noise that sounds like it’s coming from either the intake or upper intake plenum. (The sound starts as soon as the car finishes it’s “initial” warmup). The sound disappears when under moderate throttle (while driving) but as soon as you let off, it comes right back. Also, the car cuts out at higher Rpms or when you mash the throttle. Any suggestions? Thx
 
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wmburns

SN Certified Technician
Aug 14, 2009
5,894
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Houston Texas
however, after about 2 mins on a cold start there is a trumpet like noise that sounds like it’s coming from either the intake or upper intake plenum. (The sound starts as soon as the car finishes it’s “initial” warmup). The sound disappears when under moderate throttle (while driving) but as soon as you let off, it comes right back.
If you are looking for an educated guess, the noise is coming from the IAC value. Likely restricted by something. Perhaps dirty. Or have you removed any parts?

Troubleshoot IAC idle problems 1996-2004
 
If you are looking for an educated guess, the noise is coming from the IAC value. Likely restricted by something. Perhaps dirty. Or have you removed any parts?

Troubleshoot IAC idle problems 1996-2004
Decided to purchase a new iac today, and the same issue Is there. However, when I unplug the iac with either unit installed the horn sound goes away completely... thinking maybe the throttle body/intake plenum is dirty/clogged?
 

wmburns

SN Certified Technician
Aug 14, 2009
5,894
521
204
Houston Texas
These were pending codes that went away after I fixed a vacuum leak coming from the upper intake plenum going to the cold air intake.
IMO the key to the noise can be found in "the cold air intake". Did you ever wonder WHY there are such weird shapes and odd sizes in a factory intake? The reason why is to reduce the noises caused by Helmholtz Resonance.

It can't hurt to check the throttle body for build up and clean if necessary. But if you really want the noise to go away I'm afraid it will be necessary to go back to a more "stock like" air path. Especially in the IAC bypass air path.

Helmholtz resonance

It might also be helpful if you knew the IMRC position to see IF the IMRC position has an effect on the noise. But how to get this information? How about with a powerful ODB2 scanner? Here's some information on a very powerful ODB2 Windows based scanner.

ForScan ODB2 scanner w ELM327 USB
 
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Hey, sorry for being MIA, I was on vacation in heber AZ for a while. I recently scavenged my junk yard for the stock intake and threw in a new MAF (came with the intake for the same price). And there is no difference. The sound is just slightly muffled. I did come to a realization while on my trip. The kid I sold the car to decided to rattle can the entire engine blue when he swapped in the new motor. Wondering if he maybe “masked off” the plenum holes and never removed the tape or whatever from the iac bypass air path. It’s a small hole in the throttle body/plenum so it may have been easy to miss? If he started the car with that still there, it may have been sucked into the plenum and got stuck? Any advice on how to diagnose? Should I run back to the bone yard and pull a throttle body and plenum? Thx!