coolant temp guage ram

1998 Ram 1500, 5.2L, 210k miles The temp gauge on the dash goes from 210 to 280 very quickly. Usually at 260, and by 280, there is a big noise from behind the glove box. The noise is hard to explain. It sounds like water surging through the heater core, and/or something flapping. It lasts...

I just had a new thermostat put in and my mechanic must have forgot to plug in my temp sensor because I have been driving my truck around all day and the temp guage hasnt moved from ice cold. I cant find the sensor to plug it in please help. Thanks!!!