They way that you are making it sound is that you are low on hydraulic oil. Make sure that you use a bobcat hyd oil or 10-30 engine oil, not hydraulic oil. The hydrostats in those machines are pretty much bullet proof, been around bobcats now for over 10 years and still have not scene one take a dump, unless someone tore into thinking there was something wrong. I would change all your filters and don't forget the case drain filters. when taking those apart they will tell you alot. if you have alot of brass or metal shaving in them, you can see it.
For me e-bay was MUCH cheaper than my local dealer. It never hurts to give them a call and check up.
Depending on the vintage of your machine, there may be a hidden filter. Is there a square block on the top of the hydraulic pump? or is there just a big fat tube that runs to a block on the left (when looking from the bucket in).
Noise is normal from the hydrostats. Try increasing your revs, mine groans a bit with low RPM but if i run it close to full throttle it runs smooth and no noise unless i get stuck in to a pile of dirt.
If i were you, i would change all fluids as ken said and clean the chain case out. You get water in there all the time.
If you need any tips or advise just yell, i'm in the process of working on my 3rd 743, the previous owner seems to have run it on hydraulic oil and not 10W30!!! it seems ok still, but the hydrostatics under load feel different.