is200 (2002) rpm fluctuation

[iffy]

Hoping someone can help. While stationary, I can hear - not necessarily feel  - RPM's fluctuating. I can see the rev needle moving up and down in tune with the engine sound. The fluctuation is very smooth and takes around 4 seconds between fluctuations hence why it isn't noticeable. Any ideas what could be wrong and where to look? 

[The Transporter]

Is the air con running? the engine will adjust itself slightly as the clutch engages..

[iffy]

Sorry for late reply. The problem is not related to aircon use. I did though notice two air leaks - one at the induction and another at the air filter housing. I'm going to look in to that and see what the script is. Although I don't those leaks would cause much of an issue