There has been a variety of suspension related issues with the ’98 and ’99 ML’s. From what I’ve heard these are more squeaks than pops. My MB dealer has a sound testing tool where they put sensors all over the vehicle and use these to pinpoint the location of the noise, then do a visual inspection from there. This, I am told is a new tool in the arsenal, and I’d guess that not all dealers have it. Find one that does and you will be more than half way there. Also, check on Edmunds
http://townhall-talk2.edmunds.com/WebX?50@8.k8a8akOWc71^1@.ee95173 for other folks with MLs.
Regards