Verify that the source volume is not too loud for the speaker, and reduce the volume if necessary.
Verify that any panels surrounding the speaker on the vehicle are not rattling.
Verify that the source device and the amplifier, if used, are connected to the speaker terminals correctly.
If the speaker is connected to an amplifier, verify that the input level of the amplifier is matched to the output level of the stereo. For more information, see the manual for the amplifier.