I've had this odd kind of growling buzz in my conversion van forever.
It happens when I punch it in a low gear, for a long time just in second
and my very low 1st, and it normally happens only with full throttle.
I've looked, intermittently, at lots of stuff..thinking it might be
something to do with motor mounts, close clearance to the body work...like
that.
   It recently started doing it in 3rd gear also...getting 'worse'...still
only when the engine is 'on the cam' and being pushed really hard.   Well,
I had some time today and put the van up on stands, pulled the wheels and
began a real concentrated look-see.  Cutting to the chase, it turns out my
transmission mount is about shot...correction, it is shot.   With a large
screwdriver (pry bar) I can lift the nose of the tranny into contact with
the cross member it is supposed to be isolated from by the rubber
transmission mount...The rubber is cracked, but you don't see it till you
put a jack under the tranny or lift it up somehow.....which is probably
exactly what happens when the wheels get a lot of turning force.....the
nose of the tranny wants to lift up....and mine has been.....

   Being a poor working guy (again, sometimes) I ordered a new one of
probable Eastern Origin for little money.  Now for the first time since I
got my van, I can go 'burn rubber' with the kids at stopsigns...