Clearly, there's the obvious after the fact method (which I want to avoid), but I'm trying to determine when you know it's time to get it done. I'm driving a '91 560SEL on the weekends once or twice a month - probably about 200 miles max a month. She has 148,940 miles on her.

After she's been sitting for a bit (usually two weeks at a time), when she starts there's a pinging noise that I've been assuming are the valve lifters or something similar as it usually goes away when she gets warmed up. This morning though, that 'valve' noise has hung around. I drove her about 12 miles this morning and it was still 'pinging' (like that noise from low-octane fuel) when I turned her off.

It suddenly dawned on me (and since I don't know any better) that this might be the timing belt telling me she's ready. Thoughts?

__________________
Craig

2011 e350 Wagon < Grey on Grey/Brown. I kind of think I'm cool driving around in this thing, but I know I'm wrong.
2007 e350 4Matic Wagon < Black on Black
2003 e320 Wagon < Black on Oyster. Dog Hauler. Replaced at 170k. Great car!
1990 300SE, Black on Black < Totalled
1991 560SEL, Black on Black < Current, sits around a lot
1972 280SL < Sold, but not forgotten.
1972 280SEL 4.5 < Gorgeous car. Lost to a freak accident.
1985 300TD < "Merzilla". Put 300k on her.