Regarding Erik's vibration problem.

During the initial flight testing of a friends IV the test pilot reported a loud "raspberry" noise and vibration at speeds over 170 indicated. An investigation under the cowling showed that a portion of the silicone air seal was prolapsing under pressure, causing the noise and vibration.

Erik's vibration seems to be related to speed and not power so it likely relates to a component that changes as a function of speed.  The two areas I would look at first are engine compartment component interference and control surface "flutter".

In the engine compartment, you must consider that the cowling changes shape with speed as the differential pressure increases. This can cause engine components to come into contact with the airframe. Also, the engine mounts sag over time, which may be the reason the vibration has only developed recently.  Have a look at the spinner to cowling alignment. Has it sagged? It may be time to shim the mounts or replace them. Also consider that the cooling air flowing through the engine has drag and this drag results in a pressure differential between the top and bottom of the engine.  The thing is that the area the pressure is acting on is large, around a 1,000 square inches, so even a small pressure drop of  1/4 psi will develop 250 pounds of down force on the engine mounts. Add the prop "pulling" the engine with a few hundred pounds force and you can see that the engine does not want to stay put.

The other thing to check is if the trim tabs could be fluttering. Does the vibration go away if you drive the trim tabs "out of trim"?

Let us know what you find.

Regards
Brent