As said, I'd suspect the shavings as a water pump problem. A freeze plug could be a (unrelated) problem with weeping, it does happen, the previous owner blew one out on my GTA and the replacement still weeps ever so slightly due to a lazy bum install job. Any point where the system is no longer sealed, lowers the boiling point of the coolant.
Before you go yanking the heads, get yourself a compression guage to confirm a head gaslet issue.