Chances are the clunk is the AC compressor engaging. Mine does it. Do you have manual or Automatic A/C? If you have auto it engages the compressor as soon as it starts, the a/c oil goes to the lowest and coolest point in the system. When the compressor is engaged it pushes out the oil and causes a grunt. Mine does the same thing. Make sure the A/C is off before starting it and see if it stops. If it does then thats it.