[rotorwrench]

Most everything can resonate if the right frequency of vibration is induced. There have been a few problem spots on some of the helicopters I work on where certain parts will resonate at normal use rpm settings. The only way I have found that works is to change the tune of the offending part by adding weigh to it. This changes the frequency of vibration that will make it start to resonate which is, in effect, detuning the the part. It's usually long spans of sheet metal or tubing that have only a light duty amount of support in the span. On the helicopters, it's generally one of the stabilizers that has a trailing edge tab or one of the landing gear tubes. On a car it could be an inner fender panel or a fan shroud. If the hood has any oil canning from previous damage that stretched the metal there then it can resonate too. In cases like that, shrinking the metal will tighten it back up since adding weights would likely be visible. A loose air filter or horn trumpet might also resonate. Loose parts just need to be tightened back up in some way.

If it is something loose inside the engine, it should rattle all the time at any rpm setting.