S320 1997 Engine sound

I have an S320 1997 model,
When My Car is in the Nutral, I can hear the frequencey of the engine voice starts to go up and down (Hmmmmmm Hmmmmm), the sound is very very small , and also I can see the RPM indicator goes slight up and down , very small movement... please advice.... recentyl I have changed the spark blucks , the enjications, cables ,

pls send me an email to [email protected].

check at intake manifold for air leak.also check wiring and muliplug connections,(prone to rot out)