Buddy of mine has 4 cylinder toyota tacoma. His radiator fluid level keeps dropping but he says he is not experiencing a leak. Now the motor is knocking. What could the problem be?

 

Check the coolant for oil, as well as oil for coolant. This would be a head gasket, but the knocking is a little harder to diagnose.

 

Ditto to above.

Coolant doesn't just disappear, it has to go somewhere. Tell him to pull the spark plugs and look at them. If there is one or more that is super clean (ie no brown combustion deposits), you know you have a head gasket leak in those cylinders.

 

how much coolant is being lost ,and whats the time frame.remember oil and water dont mix ,so it should take a while before the coolant shows up in the oil pan. does this make sense.

 

Check the weep hole on the bottom of the pump--sometimes it leaks from there but you dont notice--run a finger under it and see if it is wet.

Knocking--Time for a 3.4 upgrade?

 

Probably a massive head gasket leak, water in the cylinders, hydro locking the motor, hence the knocking.