My 96 E300 makes a similar sound any time the engine is under load but not at idle under any circumstance. The link to the intake manifold post is interesting I plan to check that today. I suspect the injectors on mine I just have not had a spare $600 bucks to get the parts lately. I changed the belt dampener on mine it helped but that sound was different than the sound while traveling down the road. I too had changed the delivery valve o-rings and copper seals recently and have been wondering if I could have done something wrong. My engine runs and idles perrfectly which is quite puzzling. Keep us posted as to what you find I will be especially interested.
