Oversized exhaust, headers and/or different mufler.
Check your cooling fan motor. If it is no good when it goes on it will make a pulsating sound.
it will make a puff puff sound while idling.
By adding a dual exhaust and/or exhaust headers either short or long tubes for sound and performance
I sat in my car in the driveway with the car idling, waiting for the kids to come out.
Go with a 40 serious flowmaster 3 chamber muffler. That will make it sound deep and powerful!
They do not quack but rather a throaty whisper. Once you hear the difference, it's obvious.
Without hearing it, it is hard to say. When does it occur, does it vary in pitch or volume, while idling or only when moving. More information is always helpful.
I have a screeching sound in my pick up when it gets up to 2000 rpms. Does not make the noise while idling and starts fine.
ford make the mustang
You can swap a 1997 Mustang V6 into a 1968 Mustang. You will need to make several modifications to the 1968 Mustang.
Slp mufflers (if you could call them that) are what you need. If you want your exhaust to really sound tuned, add some long tubed headers, with an x or h pipe also. Go to You Tube and listen to Mustang Slp, or Mustang Flowmasters and you can get an idea of what setup to run.
Transmission is low of lube.