Unwanted string vibration

[hammer]

Here's the scoop. I've got a custom Signature bass with a maple neck that has 2 purple heart and one ebony stringer. I've been playing the bass for approximately 6-months and never experienced any idiosyncrasies or problems with it until the other day.  This involves a significant (i.e., very easy to hear at practice volumes) unwanted, vibration of the open A string whenever I play any of 3 notes (F, F#, and especially E) on the G string. This doesn't occur on either my SC or my Distillate and I've not noticed it happening any where else on the register. Tuning and intonation are spot on so I don't think they are the trouble. Any ideas as to the cause of this or a way to correct would be appreciated

[5a_quilt_top]

Are you sure it's the string?
 
I had a similar issue with my Spyder and discovered that the source was actually the cavity covers on the back of the bass rattling against the body wood.
 
I placed a bit of padding in strategic locations on the plates - problem solved.
 
I've found that Alembics are very resonant, especially at certain frequencies. In this case, all of the A's on my Spyder are more resonant than the other notes.
 
If the covers are not the source, you might want to check anything else that could vibrate when excited by the resonance of the neck / body.

[hammer]

It's definitely not anything other than the open A string vibrating.  There's no rattling sound. Just a clear A that I'm hearing (and of course feeling and seeing).  Yes, the string is vibrating so much that you can actually see it vibrating. It's almost as much as if I actually plucked it. But of course it's doing this when no part of my hand, fingers, body etc. is anywhere near the A string and only when I am playing those 3 notes (most clearly when I'm playing an E) on this particular bass. It definitely appears to be a resonance thing, but it's a bit of a pain that I have to remember to place a finger on the A string to mute it whenever I'm playing those three notes on the G string.