Ok, so this noise literally is funny!

Remember as a kid taking a baseball card or a playing card and taping it to the rear trailing arm on your bike so it would hit the spokes of your wheel and make that "rat-ta-tat-tat-tat" noise? Well imagine that sound but coming from your Mustang and you are doing 60mph down the freeway.

Here is the story. I'm cruising home on the freeway and all of this thin styrofoam is smashed right in the middle of the lane. I'm thinking to myself "cr@p if that stuff hits my car I'm going down in a ball of flame!" Luckly I swerve and avoid the majority of the styrofoam flakes. Disaster avoided.

A little later, as I get closer to home, I turn down the Shaker, from the ear drum exploding volume I have it turn up to, and I hear a flapping noise. "Hmmm, I must have caught a plastic bag someplace." I stop into the local grocery store to get some milk and look under the Mustang. Nothing is there. Nothing in the wheels. "Huh, ok that's weird." As I begin my half mile trip home I begin to hear "thap-thap-thap-thap-thap" and it speeds up as I go faster. I think... "Ugh! I caught one of those styrofoam flakes in the wheel." Not only that but the people at the bus stop on my street can hear it as I "thap-thap-thap" past them. Not a very good representation for Mustang. "D@mn it, my cool factor just hit zero!" Their looks of bewilderment don't help any.

When I get home I throughly look under the entire underside of the car and find nothing. The next day my wife and I are driving over to our friends house and of course the "thap-thap-thap" is nice and loud. But, as I accelerate onto the freeway we hear "ta-ta-ta-t-t-t flump" and silence. I look in my rearview and see a fleck of white on the road behind me. "DOH! it was that styrofoam." I say to my wife.

Only thing I can figure is that a chunk got stuck in the center tunnel right on the forward drive shaft. And, it was flapping against the U joint until it got dislodged by hard acceleration.

I'm so glad it was only a chunk of styrofoam and not something broken in the driveline.

 

It's always good news when a problem goes away without requiring any work. Fortunately, you had a suspicious idea of what might be causing it. The only similar example I can think of happened when a piece of gravel get stuck in the front brake caliper on a Honda. It sounded awful. At least it only required pulling the front wheel to find and remove it.