Any idea what is causing my problem?

So driving home today from work today in my 98 e39, I get a block from home and hear a jingle jangle type sound, of metal jangling on metal. I immediately pull over. Gauges are all fine, car is running fine. Get out look under the car, see nothing hanging down. I almost expected to see a tie rod or something hanging there. I limp the car the block home.
I get out and see a huge puddle of antifreeze under the car. Oh crap.
Lift the hood, I see that antifreeze has hit the fan and sprayed everywhere under the hood. Squeeze the hoses to and from the radiator and they seem solid, nothing spraying out, can hear fluid in them. Get back in the car, check my gauges, nothing hot. Start the car, it runs fine, still not running hot. I checked the fluid in the radiator, its under the cold line, despite the heat gauge being dead on in the middle and just driving 8 miles. Nothing else looks out of place under the hood. So what do you think this is? Blew the water pump? Or gasket?

Check your expansion tank.

Metal clanking sound?

Water pump bearing given up?

I would use test #7 (KTMP) and keep a close eye on it while your driving for a while. If it goes past 110c, it's not a good sign.

Inspect cooling components closely.
White residue indicates leaks on cooling parts.

Sounds like a water pump in the initial stage of suicide ..... Resulting in the immediate death of your wheels. You got fair warning..... Park it and investigate carefully . Do not use your key as a diagnostic tool .