Noises problem
1999 Toyota Corolla 4 cyl Two Wheel Drive Automatic 160K miles

I have a problem with my 99 Toyota Corolla, 160K miles.
Over the past few days, every now & then I hear some weird noise after I start the car, as if something is hitting the motor / engine / fan thats circulating (similar to noise when u r blocking a fan)...I tried turning the car off and removing the keys and yet it feels as if the engine is still running...which was very scary! and the 'obstruction noise' doesnt go away...

I have already diagnosed with two mechanics and obvuiously payed them as well...but bothe their theories sound weird..

1st guy says its bcoz of ignition prob which doesnt stop when u remove the keys...Other says its engine belt (& he showed me some cracks in my engine belt) and says thats the cause...

A 3rd theory didnt seem correct upon inspection...that fan belts have slipped...

The car runs good except for this prob...Can u suggest what else can I chk to confirm wat prob it is...?