There are MANY things you can do to reduce noise on your 98C!!!!

There are instructions you can find with a search on Google that give GREAT DETAILED instructions on making an almost silent 98C.  As others have said, due to the mechanical nature of a paintball gun you can never TOTALY silence one but you CAN make a GREAT reduction in the noise your marker makes with a SMALL investment and a little elbow grease. I have done a great many to mine and if you are more than 30-40 feet away you will hear the sound of a ball whizzing by you louder than the sound of my 98C.  YES, the sound a paintball makes whizzing through the air is LOUDER than my 98C at 40 feet.  At 50 feet you simply can NOT get a deffinate bearing on where the sound came from.  (during sound tests we shoot past the person so they can get a good sound comparison between the ball and the marker fireing, during the game you will just hear the pop of the ball on your chest)

First is to put rubber washers or a layer of neoprene between the grip and your asa or drop forward.  This will greatly reduce tank ping and makes a GREAT deal of difference.  You can test the difference you can expect by just holding you tank tight to deaden the ping, check this one for yourself FAST that way!

Second is to reduce the noise of your internals!! Polishing them is first (do a search, there are many sets of instructions for this).  Another noise reducer for internals is to put a bit of lithium grease on each spring inside to reduce noise from spring vibration. Some advise a stripe of silicone on the springs but a good gob lithium does the job and you do not have to worry if some gets in other parts of you internals!  You can also put a generous gob of lithium on your sear, it will help also (EVERY little bit helps). Also get RID of any "noisy" upgrades like the cyclone feed or responce trigger if you have one.  DO NOT use an electric hopper or ANYTHING that makes noise on your marker if silence is your goal!  I EXPECT that if silence is your thing then shooting 20bps is NOT what you want in a marker anyway, but just remember, if it makes noise, LOOSE it.  The stock 98C will shoot 8-10bps witout ANY of that extra noise!!! (if you have a double trigger and PRACTICE walking)  MOST people will NEVER shoot that fast with a mechanical marker anyway and MOST people will NEVER shoot fast enough without an e-grip to out-run the 98C stock out of the box.

Third is to get a Rocketcock II.  It not only reduces noise by covering your cocking knob port but has the added benifit of keeping dirt and paint out as well!!!  When you put it on you can use a little silicone or "form-a-gasket" to create an even better sound reducing "seal".

Fourth (and most expensive) is to go low pressure. I hear that low pressure reduces the noise a great deal over co2 and even hpa/nitro.  I have yet to test this one due to $ restrictions, but the future will see, I want the most SILENT marker you can create!

Not last (there are more, just search for them), but most important is your barrel!!!!!!!!!  The stock 8 1/2" sounds like a CANNON with all the noise it makes!  (The 98 flatline is just as bad, it is LOUD) Get the longest barrel as You can stand to play with. (well ported for silence it will disperce the sound up, down, left and right, instead of just foreward) The longer the quieter.  You will not get any distance or acuracy benifit from a longer barrel but the LONG jobs do not harm distance or accuracy either as long as you get one sized to the paint you prefer.  I have a 21" barrel ported for silence and it is just as acurate as my 14" progressive but it makes less than 1/4 of the sound!!!!! (AND you can stick it out through THICK brush too!)  If your style does not play well with a barrel that long just go with the longest one you CAN stand, 14", 16", or 18".  Every inch of ported barrel reduces the forward muzzel sound of that expanding gas.  Again, test this one for yourself.  You WILL be amazed at the difference.  Beg borrow or buy one and test it!

GOOD LUCK MAN!!!