1999 Subaru Forester Clutch problem?

I have a 1999 Subaru Forester:banghead:

For a couple weeks I've noticed after I start it, when I shift into first gear it will shift fine, but almost immediately after shifting, there's rumbling/vibration that lasts about 2 seconds under the center of the car almost immediately under the shifter. At first I thought it was something heavy rolling around behind the passenger seat.

It only happens when I put it in gear from a stop position.

I replaced the clutch about a year and a half ago.

Had the sparkplugs replaced about two months ago.

Other than the breaks squeaking a bit, not issues to speak of - just can't figure out what the rumbling/vigrations are caused from and if it's something I should worry about.

It has about 140k miles.


ANY ADVICE/HELP WOULD BE APPRECIATED!

THANKS!