Strange rattle under instrument panel

[4059]

A strange rattle has appeared suddenly in our 120. We have narrowed it down to an area near the center of the top side of the instrument panel ( right behind the windshield). When heavy pressure is applied by the palm of a hand on the top of the panel it disappears. There is no rattle at idle RPM but as soon as the engine is turning at about 1000 RPM we begin to hear it. The rattle continues over the rest of the range RPM but seems to peak around 2000. We can hear it in flight as well as on the ground. It seemed to appear soon after departing a grass runway which is in pretty good shape with just a couple of rough spots. Just was probably just a coincidence though but that is when we noticed it. We have checked the tachometer cable, windshield screws, firewall,(both cockpit and engine side) and all the various lines and cables under the panel but nothing seems to be touching. It almost seems like the top of the panel has somehow gotten into a bind and and as the airplane's natural engine vibration (which really has very little) shakes the panel ever so slightly we can hear it. Even with noise canceling headsets on the rattle can be heard but you cannot hear it outside the airplane. We are scratching our heads but wanted to see if anybody has seen anything like this.

[6643]

There is a stringer in the top center of the boot cowl that runs from the top engine mount attach point to the instrument panel. It's item 9 in figure 17 of the parts catalog. I suspect it's cracked.

[4059]

Thank you! We will take a close look at that stringer

[6277]

It could be the instrument panel cover vibrating against the stationary panel. I had a similar noise. My panel hooks over two prong type fasteners at the top and is held in place by two screws at the bottom. One of the prongs broke off. My temporary fix is a small piece of cut vinyl/rubber tubing to deaden the vibration.

[6898]

Just a thought but check to see if the windshield has any dust or grinding marks on the lower part? Maybe has to be resealed?

[8405]

First off thanks for all the replies!
Anyway a little digging later and it turns out the stringer was cracked! I'm not 100% sure it's what is causing the rattle but it needs to be fixed regardless. And suggestions on how to make this repair? I'm leaning towards removing the whole assembly and either making a doubler or fabricating a new stringer.

[6643]

I think by the time you finish fabricating and fitting a doubler you could have replaced the whole thing more than once. I'd make or buy a new stringer.

Just to give credit where credit is due, back around 1990 my uncle spotted an article (maybe by Bill Rhoades) in a magazine, probably Vintage Flyer, about just this issue, and pointed it out to me. My long term memory is still pretty good...

[8405]

Just a follow up in case anyone else has a problem like this. While I am happy that we noticed the crack in the stringer it wasn't the cause of the rattle. Turns out the engine mount was cracked at the center mount point!



Kinda obnoxious considering that the mount was just repaired two years ago but at least the problem has been found. Thanks again to all those that replied!

[6643]

Good catch! Most likely they are related. The stringer is on the other side of that mount point and helps distribute the load. (But, no doubt, you already figured that out.)

Interesting that pressing on the glare shield quieted the vibration.

By any chance, was the mount repair because it was up on its nose?

[8405]

That's kind of what I was thinking too, that the two cracks are connected. I guess maybe pushing on the glare shied flexed the firewall just enough to push on the loose part of the mount enough to quiet it down some.

It wasn't on the nose anytime recently. The mount was sent off at engine overhaul for a precaution, a crack was found at the same mount point but it wasn't nearly as apparent. It does have some distant damage history, a ground loop in the 60's and a not insignificant portion of its life it was operated off of a fairly rough field. Colorful history like many of our airplanes.