My 75 F250 was doing the same thing. My truck had not been started for a long time, like 8 years, when I got it. The heater fan would just howl when I switched it on so loud you couldn't talk over it. Sometimes intrmitent and sometimes non-stop. Then once in a while it would be quiet for 10 minutes and then suddenly start screeching so loud I jumped an almost changed lanes. I tried squirting some oil into the squirrel cage hoping it would hit something. No luck. As a last resort I went into the engine compartment and sprayed the casing of the fan motor protruding from the firewall with WD40. I soaked it with oil. The next day when I turned the fan on it chirped for 10 seconds and has not made a noise since. Its been about 8 months and is still working fine.