Hi again. Our research at www.wetstart.com
has dealt with this. We researched the oil pressure lag build-up behavior of engines uppon start (hot/cold). One of our research vehicles has been my own 91 300E. Occassionally it would make the noise you described (other vehicles will have hydraulic tappet noise) and it was correlated for the most part to oil pressure lag. As part of our research we kept that engine permanently primed with oil and all noises are gone. Using this method we can make fossil oil "move faster" than the best synthetic (flow speed that is) by virtue of the "fossil" coming to rest inside the oil galleries (primed). So based on our observations:
Check your oil filter for proper anti-drain back valve function (just replace with a new one); Sometimes the pump (????) would loose partial prime and take longer to pump; Obviously viscosity will make the fillup of galleries more difficult (a reason to: change it hot and change it often......I think somebody has said that before many times). More research results on this at www.uspto.gov
(#5,743,231; 5,782,315; 6,349,692) if anything this site is geat for fun and research. Later.