Can someone identify this sound?

I've got a 99 Nissan Altima with an odd sound to it while accelerating.
It's a
regular thrumming, sort of "fluttery" sound. My 94 has a very similar sound
as well, but a bit more harsh I think (due to less sound deadening maybe),
so it could very well be normal. I'd really like to know what causes it
though.

I went for a drive and recorded the sound inside the cabin, it's a bit hard
to make out but any time I accelerate it's there. It's most noticeable
under moderate acceleration it seems.

I've got a short clip and the full length at
http://www3.ns.sympatico.ca/pmason/altmp3/.

To me it almost sounds like a resonance that is happening somewhere, in the
exhaust manifold maybe or some part of the engine bay. I had a Civic a few
years ago that due to an accident and repair had the engine mount touching
the frame of the car and unless I wedged something between the two parts, I
would get nasty sounding knocking/vibration while driving. Maybe this is
the same sort of thing. It's almost like some metal part of the engine is
vibrating against the hood, or the frame, or something.

Also, I get a slight tapping for 1 or 2 seconds at startup when the engine
is warm (but has been sitting for maybe 20 minutes). You can make it out
faintly at the start of the longer recording here. I've been told it may be
a stuck lifter, so is there any chance that that could account for the
noise? It seems to happen at about the same frequency or tempo as this
sound, taking the lower idle rpms into account. I would expect something
like that to have a lighter sound to it
than this does, but I'm no expert. I've been telling myself that it's
happening (revolving) too quickly to be something to do with a particular
cylinder or valve, but again, I'm just guessing.

I know it's pretty hard to make any guesses, educated or otherwise just
based on a couple of hard to make out samples, but I would like to hear any
theories just the same.

Thanks!