If the level is correct and the warning persists, then the sensor must be at fault, that is not a code you would have to clear. New sensors are relatively inexpensive and I think a pretty easy DIY fix.
Agree with Andy, aftermarket parts fine for most jobs, but transmission oil must be the proper stuff, though you can still get cheaper than £300. It will exasperate you at times, but when it's running well nothing finer!
Does the noise persist when driving? Most knocks are suspension or wheel bearing, the latter may be a more constant grinding noise. First thing is to remove the wheel and check for any obvious suspension issues. Difficult to assess wheel bearings as the usual method of looking for play when rocking ...
I fitted one that goes around the number plate on my old X300. Ok it only has 2 sensors on it, at either side of the number plate, but it worked well and was easy to fit, splicing it into the reverse lamp wire. I am pretty sure you can still get them, have a look on eBay.