Exhaust Fumes in Cabin while idling.

That’s fairly unusual, unless you are frequently idling in an enclosed area.

Had the car ever been in an accident? Rear-end hits are good at compromising the exhaust pipe...

 

I would NOT delay at all.
Get it checked out ASAP.
Exhaust fumes contain Carbon Monoxide which can be deadly.

 

where did you buy it? i would take it back for an inspection.

how many miles? do you know the history?

this is unusual to unheard of, especially in a 2 year old car.

 

Also check the weatherstripping that seals the back hatch when it’s closed.

While you are looking under the car to inspect the exhaust system, also look for other possible issues.

There are probably some rubber plugs under the car that snap in to close various openings, make sure there are no obvious open holes to the interior, or spare tire well, etc. It’s common for body shops to forget these plugs after repairing a rear collision.