Buick Century-Heater Fan start making noise after some time

Buick Century 1999
6 cyl. 3.1L
90 000 miles.

The car starts and runs fine, but when the heater is turned on, after a few minutes I hear a noise, which is kind of loud. If I turned the fan off, the noise goes away and does not come back even if I turn the fan right back on, but comes back after a few minutes again.

Thanks