Over revving when idling and sounds like a hole in the exhaust but no hole?!

[Lisa00]

Hi everyone, 

I have a problem with my '13 RAV4 it sounds like there is a hole in the exhaust but there is no hole, engine seems to be over revving when it's idling and in low gears, however it does improve if I switch the heater/air con off! Does anyone have any ideas what it might be? 

[IanML]

There may be a leak from the exhaust manifold gasket.

It is normal for the idling revs to be raised when the aircon compressor is running.  What rpm does the tachometer show when the engine is warm and the aircon is on?