Still a strange rattle at1500 rpm

[xavier]

After tightening belts and listening to the alternator and power steering pump with the solid metal bar(stethoscope style), I still cannot find the sorce of a rattle I get at 1500 rpm. Above and below this there is no rattle. I lay under the car while my wife kept the engine at 1500rpm. It is definitely at the front of the motor near the fan. Any ideas?