I have a coolant leak that none of my mechanics can find. What could it be?

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I've had a coolant leak issue for the past year. First they replaced the cap. Then they replaced the heater control valve. Recently, the temps dropped below freezing and I noticed a puddle of coolant under front mid/passenger side of my car.This is where the puddles always appear. I checked the expansion tank and it was full. Took my 1992 Previa in and they did pressure tests and couldn't find any leaks.The radiator was full. They checked the water pump and thermostat and all showed no indication of leaks. They sent me on my way and it didn't leak for 3 nights. The next day the temp dropped again and there was a large puddle under my car again. I checked the expansion tank and it was empty this time. I filled it and took it back in. The radiator was low but again they could not find or reproduce the leak. I had an oil change while I was there. They sent me on my way. The engine now knocks and shakes at stops and when reversing. It eventual stopped after about 10 miles. I went to work and the heater was running hot. I left work about 7 hours later and half the car ride home (about 15 minutes) was fine but then the heater started blowing cold air and the knocking returned. Checked my car this morning and sure enough the reservoir was empty again. I took the car to a different mechanic and they too could not find any leaks. Anyone have an idea of what could be causing this issue. I'm at my wits end.