E 300 D occasionaly will not engage starter

[nelbur]

Our E 300 D usually starts fine. Occasionally when we turn the key to the start position the starter will not engage. Instead we get a soft continuous beep or buzz. We will typically try both of our keys multiple times with no change. eventually it has always started, so far, but it is getting worse. I thought I would have to have it towed tonight, but it finally did start. Does anyone have an idea as to what is going wrong?