Had a bit of a squeaky noise from the front of the engine. A quick look didn't show anything and I'd thought it was perhaps one of the pulleys feeling tired. As a result, driving was limited to 'absolutely essential,' and the 500SE pressed into service for shopping. (Mrs M hates going shopping in that!)
Today, pulled things apart and found the aircon belt hanging off the front of the compressor. The smooth side was polished to a lovely sheen, and the ribbed side cracked all the way around, although nothing is missing. We were very lucky - if it had caught on the fan things could have been very bad indeed.
These belts never get changed. BMW don't stock them in NZ, and RealOEM only shows a kit comprising belts, pulleys, and so on - p/n 11282339103.
However (and for anyone who might need to know) the belt is a 4PK 968. If possible, get a 4PK965 as the 968 has the tensioner almost fully extended; the 965 doesn't fully compress the tensioner but does give a teensy bit of stretch leeway.
While you're there, you may want to check the main drive belt, another 'never gets changed' item. Again, RealOEM shows this as a part of a kit, p/n 11282339099. Again, you can buy the belt separately at ~$90+GST from BMW (they have 1 in Auckland, but more often than not bin them when shelf life expires), or from a factor as 6PK2090.
The aircon belt can be replaced from under the car (remove the engine undertray), but the main belt will require fan shroud removal, which requires removing the fan.