Hi,
if you want to check the transmission oil level you can do it if you'll have an allen wrench that is long enough. Besides the upper opening isn't easy to open and close without removing parts. I tried that and ruined the screw. So i recommend to remove the shield below the transmission. To do that you have first to unbolt the stabilizer. Please check the workshop manual for detailed torque specs and how to's.
If you check the transmission oil level the engine and transmission has to be fully warmed up and the car has to be levelled. So the best is to drive a while and the put it on a lifter. You can't check the oil level if the transmission is cold, because than you'll get the wrong level. If the transmission is cold, the only way is to change the complete transmission oil and fill in the exact amount.
Do it yourself: if you have the tools and experience in working on cars think it is no problem to do it yourself.
Please use a approved Porsche transmission oil. Wrong oil can cause big problems. Take a look:
http://986forum.com/forums/437747-post24.html
I would first check oil level. If it's OK and the noise is still there you'll have to remove the transmission and check what's the cause for the noise.
Regards from germany
Markus