When I start my car (2005 MCSC 6 speed) in the morning and put it in reverse to back out of its spot the backup sensor gives a steady tone (as if I was right up against an obstruction). If, before moving at all, I shift out of reverse then back in the sensor tone stops and all is good. It does this 100% of the time if the car has been parked and the 1st thing I do is back up. It will also happen most of the other times I go into reverse. Just an annoyance. Wondering if anyone has experienced or debugged this?

 

Its tells you the sensor has an error or is bad. They do get water in them and fail. Some options below. Mine did the same thing and I bought some time with a hair dryer to get the water out of the head of the sensor, but next times it rained, it came back.

https://www.northamericanmotoring.co...-up-paint.html

 

On thing you MIGHT be tempted to do is pull the fuse....DO NOT DO THIS.
THE POWER STEERING PUMP FAN is on the same fuse....if you want to disable the system, do it at the sensor.
One trick....when the back-up sensor you are having trouble with stops working (after you have fixed the issue), the fuse is usually blown due to a seized PS fan.....this tiny bit of info can save you $$ cause you can get a new fan and install it in 15 minutes, rather than a $1000 PS pump later....

 

+1
Also you can put your ear up to the rear PDC sensor ( 4 of them) when its in reverse and the ignition is turned on and engine not running, you will hear click click click.... Each PDC sensor will do it, the sensor that does not do it is the one that bad. My R52 did this and that how I found it. This is before i had the schwaben scan tool.