The possible causes may be:
Musty odors indicate the existence of some trapped water, usually under the carpet. This will ruin the carpet and also speed up corrosion.
Smoky smells are of various kinds. Electrical smoky smells are the most dangerous and it is advisable to shut down, open the bonnet, disconnect the batteries and seek professional help. Other smoky smells may occur when the sound or heatproof lining inside the engine space burns.
Fuel smell occurs either when the fuel tank venting gets choked or if there has been an overflow.
Sometimes, Oil or exhaust smells from the engine occur.
This maybe caused either due to too much engine oil, or if the engine oil has grown old and is burning, or if it has been overheated.
A typical sweetish smell emanates when varnish is being burnt which occurs when your dynamo or alternator is about to pack up and would, Usually be accompanied by flickering lights.
Burning rubber smells tend to come when you drive with your brakes on. Overheated tires may also be the cause.