i have a 2000 s2k and when i start it up after its been sitting a few hours or even the first few times it makes a weird squeeling/sputtering noise after the car is turned on. it also makes a noise similar to it around 3500 rpm, i was thinking it may be the belt or pulley (NOT a belt sounding squeel) but my dad says it might be the catalytic convertor due to a bad air/fuel ratio from a possible faulty/overused air filter. i just ordered a new air filter today and havent got the chance to look under at the catalytic convertor after regular running temperatures. i just bought the car last week.

 

More information is required. Open the hood, stand and listen as someone else starts the car. You will get a general idea where the noise comes from. This will greatly help someone on this forum identify your problem.

 

TCT