I agree with the other posters, 'AHKV', it is almost certainly condensation. If, as you say, the exhaust system is past it, and if the car is a 'keeper', it may pay you to consider fitting a stainless replacement. Acidic condensation can be responsible for the premature demise of most mild steel systems.
Regards, John.
__________________
Time flies like an arrow, fruit flies like a banana .....
|