It could be one of a number of things:

1. Full of gunk and will get quieter if it gets cleaned out - two ways of doing this ..

a) Try the saniflo descaler / cleaner product http://www.sanifloproducts.co.uk/products/Saniflo-Products-Saniflo-Descaler.html

b) If you are comfortable with it, to clean:

1. Turn off the electricity to the unit (should be on a fused spur)
2. Disconnect the pipes (inlet and discharge) - you may get a bit of surplus water coming out so have some sponges and a bucket ready.
3. Disconnect the wiring and take the unit out
4. I clean my Saniflo's outside with warm bleach water ..
- Wear rubber gloves to keep germs out of any cuts you may have / get on your hands
- Put the elextrical cable in a taped up plastic food bag or similar to keep dry
- undo the 'star screws'
- Flip up the plastic retaining tabs with a flat blade screwdriver and gently ease the lid up.
- I put bleach and warm water in, leave it for a while (30 mins) and then swish it around and empty out into an outside drain etc
- If you want to use an old toothbrush to get into the areas by the float switch and under the motor it does help. I also used a hosepipe in the past which did the trick.
- Also pay attention to the inlet port and the actual pipes under your sink for gunk

put it back together, aligning the seal and plastic clips should just pop on (may need a firm push). Do up star screws and you are ready to reconnect the pipes and electrics.

I use some tissue paper to check for any leaks around the pipes just to be sure!

2. The bearings in the motor for the pump may be going (i.e. have play in them) and be causing noise. This normally means a replacement unit

3. Something is in there that should not be! ... i.e. toy soldiers etc etc (see cleaning)

Hope this helps