Temp gauge at 120C-but no clue! Help

[Gary Ganaway]

No problem with temp gauge or overheating previously since I bought car in May. 1995 SL500, 65K. Went on drive tonight, stopped to fill up with gas, then after about 5 minutes happen to look at temp gauge and it was at 120!!. No coolant smell or leaks as I stopped and checked under hood. Immediately drove home, about 5 miles, and checked under hood again. No leaks but I do hear a hum like sound coming from, what sounds like under engine below exhaust manifold on passenger side. Engine has been off about 15 -20 minutes and I still hear this humming sound, like a very small motor noise. Aux fans were not on before or now with engine off.

Any ideas what to look for here? This is first benz I have had where I have seen temp gauge go up like this. Gaug does work as when I turn off ignition the needle drops down to bottom of gauge as it should. Thanks