I own a 2001 DeWoo. It makes a rattling sound when the engine is running which seems to be coming from underneath the car -- like under the driver's seat. The engine itself is still quiet, however now when I am sitting at an idle the temperature gauge goes midway up between the 1/2 mark and hot. When I begin to drive again, it goes back down to normal. I checked the oil and it is okay, but the coolant reservoir doesn't show any coolent. I can't get the top off the coolent reservoir, so I am not driving the car right now.

Optional Information:
2001 Lanos 4 cyl.

Already Tried:
Checked the oil and looked at coolant reservoir