Jimjams. We have one of these and had to replace it just recently after more than 12 years when the motor gave up the ghost. I,m afraid it did the same as yours.....very loud then very quiet.
I'm afraid the service is quite expensive but if you are handy, it is a reasonably easy (but smelly and dirty) job to do yourself.
Start off by emptying the toilet as much as poss with a soup ladle. The get as many bath towels as possible and pack them around the floor of the unit. Next, MAKE SURE THE POWER IS OFF AT THE MAINS, and disconnect the power supply. Then you have to work your way around the unit disconnecting all the inlet hoses and jubilee clips. (ours had 2 inlets, some have 3 some have 1).
Be ready with the bath towels to catch the water out of the pipes as you disconnect them, but you shouldn't get any foul water out of the unit as it has internal valves.
Take the whole thing (it is VERY heavy) into the back garden. Take the lid off. Be VERY careful!! There is a seal which must be returned exactly to where it came off so watch where it is when you are dimantling the unit.
Now tip the yucks out onto the grass/ onto the compost heap/ over the neighbours wall ...only kidding!
At this point we used the garden hose to clean it out which is quite high pressured which promptly blasted back into DHs face! DS started running aroung shouting 'Daddy's covered in pooh!'
Okay, you now need to reassemble the whole thing, plumb it in and re-connect the power. If it doesn't work after all this, it is knackered and you need a new one.
We managed to plumb in the new one ourselves aswell.
Good luck!