Rattling noise in car

[Bubble]

When a car moves at a  certain speed, htere is an annoying rattle.  If the driver speeds up or slows down slightly, the rattle disappears.  WHY???????

[adydb]

Your car?

The wheels either need to be balanced or the tracking done.

[brachiopod]

Resonance.

At a certain speed (whether mph or engine speed ie revs) the frequency of the rotaion of the wheels or crankshaft amplifies the natural frequency of, say a body panel, or another engine part, creating increased noise.

The same principle destroyed the Tacoma suspension bridge, Washington State, US, where wind of certain frequency (not a particularly strong wind) amplified the bridge's natural 'resonance', creating violent bowing that ultimately destroyed the bridge.

Most bridges are constructed and tested to avoid the natural frequency of a main component being a factor of the natuarl frequency of another component.

See also Millenium Bridge

[Clanad]

A small addition to brachiopod's explanation; an engineering term equally as valid for this phenomena would be harmonic vibration...the results are the same, a part of a structure, in this case a car, can be caused to vibrate, therefore causing a sound, by the vibration of a totally unrelated part.  The part being caused to vibrate does so in harmony with the originating vibrating part.  This phenomena has caused the destruction of aircraft due to harmonic vibration in certain controls that originated in yet other controls...

[FastBarry]

I have an annoying noise in my car, "the wife".

If I go over the speed limit, the noise starts - "Do you know you're doing 80 miles an hour" - I slow down and the noise stops. Amazing.

[Clanad]

Annoying, noisy parts on automobiles are most often removed and/or replaced...

[charliep88]

you said an annoying rattle however if you get used to the rattle you may just sit back and enjoy it . a friend of mine had the same problem and now he has had a bigger rattling noise placed in his car , remember annoying is in the ear of the beholder

[landie]

Check that your exhaust pipe is not loose.