What is a problem you might notice using your sense of smell?

A fuel leak.

A coolant leak, coolant has a sweet smell.

An exhaust leak.

A burnt clutch.

A burnt rubber smell, from a slipping accessory belt/tensioner or rubber contacting the exhaust.

A Moldy smell with the A/C on from a dirty/moldy evaporator.

An oil leak, if it is leaking near the exhaust manifold, from say the valve cover.

A smell from burnt brakes from a faulty brake caliper.

A burnt plastic smell, from plastic melting against the exhaust( wiring loom, or plastic bag on catalytic converter).

A. mildew smell caused by a water leak, clogged evap case drain, flogged sunroof drain.