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

wmburns said:

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

Troubleshoot IAC idle problems 1996-2004

Back to the basics. Check for blown fuses. Especially fuse F2.2 and F2.8 in the Central Junction Box (CJB 1999-2004 MY). Confirm key on power using a known good ground. 1999-2004 MY fuse panel schedule...

www.stangnet.com

Click to expand...

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?

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!