Any ideas of the cause
Rusty nails? Cosmetic at the worst though.
It’s a 1970 house and the stains come out of the siding seems…there appears to be weep holes in the siding but there is no discharge from them wood brickmold windows.
The north east and west side.
Maybe condensation from little or no insulation.
I have seen similar when aluminum nails were not used with aluminum siding.
Dust blows in the seams and exits the weep hoes and seams as water too blows into the seams. It is that way on my vinyl sided house.
Lack of a WRB.
Moisture staining from sheathing draining through weeps and butts.
Possible abrasion/rubbing wear from expansion/contraction. (Any shiney areas?).
Seem to be on the seams. What is common factor? Material overlap? Plus water intrusion so could be some AL overlap reaction and maybe some other material within reacting? Apart from cosmetic discoloration any other concerns?
The answer is… Recommend a licensed siding company to review/repair…