I have had a high-pitched hum coming from my '86 syncro Westy for a few
minutes most mornings, but it always went away before I could figure-out where
is was coming from.  

This morning it was particularly loud and was continuous for my entire
commute. It continues to hum for a minute or two after the motor is shut-off.
So I start hunting and pressing my ear all over the van.

I found it finally, under the van 2/3 of the way back on the drivers side. 
There is this plastic module that looks like three sphere put together.  A
wire or vacuum tube comes out of the back and goes into this "y" junction. 
One of the tube/wires goes off into a harness and the other goes into this
short cylinder thing.  It is the shape of a short, sqaut c size battery and
then out of ther back of that a tube/wire goes into the harness.

Well this little cylinder is humming away and annoying as hell!  It doesn't
seem to affect the engine performance, but when I was playing with it it idle
change a little.

Question:  What is this thing?  Is it suppossed to hum?  Should I replace it?

Help, please! Thanks,

         - Craig