Originally Posted by Scott ZHP

Some less common air/vacuum leaks on the M54... one or sometimes two small vacuum caps on the rear of the intake manifold; they split and fall off. The tubing/vacuum valve that controls the secondary air pump is also back there. Dipstick o-ring where it enters the oil pan. The vacuum tube leading to the fuel filter/FPR splits; this leak will only be visible under the car. Any/all of the CCV hoses can crack.