When it reverts to defrost that is telling me your running out of vacuum. It could be a leak in the vacuum storage container. Mine had that. Ford wanted over 1000.00 to put in a new one. It seems you have to dissemble the whole vehicle to get to it. I had my mechanic put in a different one on the firewall, works like a charm.
One other thing It could be an exhaust noise, that year was famous for that. Knocking when under load.