Any new or strange sounds coming from any part of the vehicle. Notice a clunking sound when going over a bump? A suspension part is worn out and needs replacing. Loud whine when turning? Power steering pump is possibly going out. Growling sound when driving at speed? Need a new wheel bearing. ANY new sounds coming from the engine? Source needs to be found fast to prevent possible massive repair bills.
It sounds paranoid, But I’ve saved myself thousands by diagnosing and fixing a few hundred dollar problem before it evolved into a few thousand dollar problem.
Check ALL fluid levels weekly. Inspect for new grime covered surfaces on the engine and transmission regularly, which indicates a leak.
If you know enough about cars to understand live sensor data you can get cheap wireless OBDII scanners and monitor your engine stats off an app on your phone. I keep one on my dash and sometimes I’ll plug it in while driving to asses whats going on under the hood. Once I caught a vacuum leak within a minute of it occuring this way, had it fixed within 20 minutes for 6$. A mechanic would of charged me 75$ atleast. Not to mention the cost of increased fuel consumption.