I had a similar problem on my 87 300E, the noise only occured while traveling at low speeds while going over bumps and/or turning while going over an incline or decline. It took a long time to figure out what the problem was, but it was the front brake caliper brackets. These were worn causing the posts which run through the caliper to slide, causing the groaning and bumping noises. After replacement, problem solved! Hope this helps.