One of my electric baseboard heaters makes a high pitched hum when turned on to 70 degrees, unlike the others. What could be causing this and how can I fix it?

See this previous post, it may help:

https://www.askmehelpdesk.com/electr...hum-45561.html

If not, the element may be humming inside the tubing that actually houses the heating element, and this cannot be repaired.

Is this condition dangerous?

No it is just the metal vibrating in tune to the 60 cycle hum of the Alternating Current.