It may be your balance shaft belts.  My first timing belt replacement
revealed that mine were broken.  Before then, I assumed that it vibrated
because it was a four cylinder.  After the belts were replaced, it smoothed
right out.  If your mechanic replaced the timing belt though, he should have
noticed something like that.  Might be something to look into.

I've read it here often before, drive other 944 cars. Your questions
about the character of the ride,"vibration"clearly points out the
value in doing this. No vibrations aren't normal. They are concerning
and need attention. I have a 1987 944 coupe with 115,000. and the last
"vibration" I felt ended up being cured with a wheel alignment and
balancing, not at all what I thought it would be. Had it aligned and
balanced 2 yrs. prior. What I determined is that the wheel balance is
very delicate on theses cars. When all is well, whew, no better ride
on twisties or track. Maybe faster and torque-ier but my god what a
ride. If you drive a car that has it right it might give you a frame
of reference for your car. BTW I personally don't believe any
maintenance has been done that I can't see documented. I'd want to see
a receipt for the timing belt job. Also any other work that is
claiming to have been done. If not from an expensive maintenance point
of view, I would consider planning out the maintenance that has to be
done. Rather do or pay for a timing belt job then have it let go and
have to pull the head replace the bent valves, etc. etc. etc...
Maintenance is expensive, not doing it more so.
Hope this helped.
don
"It's not how fast you go, It's how you go fast"

<snip> it makes a loud rumble noise and it vibrates a lot.  I wonder if
this is normal.

Noise and vibration are "normal" when the motor mounts are bad on a
944.

Otherwise, the engine is very smooth (when the balance shafts are
working right - belt hasn't slipped, etc.)

As others have noted, other things can cause noise and vibration as
well.

absolutely not normal.  could be several things, including a totally
incompetent mechanic - I got a bunch of parts from a nice porsche that
became a parts car because an idiot mechanic installed the timing belt
wrong - the guy selling the parts by then knew what was wrong, but he
said that he kept taking it back and the guy would put on a new belt
and say "oh, these cars eat belts" -

so, do the following in this order:

1. download the PDF porsche manual (it's on the web) and read the part
about checking the belts, and then you personally check them, or stand
there while a mechanic checks them - make sure both are installed
correctly, not broken, and properly timed.

2. read the part about checking motor mounts - usually the one on the
pass side fails first due to exhaust heat - you measure height to see
if they are good.  they are a pain to change but it's not particularly
hard, just painful.  If they are out of tolerance, then change them.

3. if engine idle is rough, check vacuum and fuel pressure - also
check air flow sensor - car is very sensitive to vac leaks

4. if vibration is at speed, check clutch, check wheel balance, check
CV joints

recent head gasket replacement suggests overheating or a broken belt
was the real cause - so you may well have some significant engine
issues to fix - not hard, a valve job is about $600 in parts and labor
if you R&R the head yourself (less if you don't have to buy all new
valves).   you could have a cracked head, cracked block, improperly
torqu3ed head, bad head gasket, etc - a compression test will reveal
some or most of this.  Also, ignition wires get leaky after about 10
years, if they are old, buy OEM replacement wires and change them and
the dist cap and rotor to eliminate ignition misfiring.

Bill

www.wbnoble.com

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