2005 Mustang GT Very Strange Noise

My 2005 mustang gt is making a very strange noise only at highway speed above 60 mph. Almost like a knocking/thumping/rhythmic rattling noise. I also notice that my headlights are flickering but ONLY when the car is making the thump. It almost makes me think it’s an electrical problem but i’ve never heard a bad battery or alternator make that noise.

To make things even stranger the noise is very inconsistent. Sometimes it’ll make the noise at highway speed and then it will go away even though my speed doesn’t change. It won’t make the noise accelerating. At lower speed or idle it also doesn’t make any noise. It doesn’t speed up or get louder with rpms. I am positive it’s not the engine

I’d check the harmonic balancer on the crank pulley to make sure it’s not coming apart



Can i test it by just trying to wiggle/move it and if it moves at all it’s bad?



Loose plastic underbody shields can make a lot of noise at highway speeds.

Another possibility might be a failing accessory belt tensioner.


That could definitely explain the flickering headlights too

Do you feel that noise as well as hear it? If you feel it…

I’d check the 2 piece driveshaft center bearing rubber isolator while you check for loose undercar panels.

One idea: Ask your shop to place a shop volt meter where you can see it as it monitors the battery voltage as you drive. Then when the noise and headlights symptoms occurs, you can see if the voltage is dropping, or oscillating, or doing something it usually doesn’t do, etc.

Long shot but I did have an alternator lock up and spin the nut and pulley off. No idea if there were an noises before.