Why does my Audi A3 engine make a humming noise when idling?

Sometimes when I'm at a stop light, I notice that my Audi A3's engine makes a strange humming noise. It doesn't happen all the time, but when it does it can be quite loud. Is this normal or should I be concerned?

Your Audi A3 may have a faulty idle control valve, which can cause the engine to hum when idling. You may want to have it checked and possibly replaced by a certified mechanic.

Another possible cause for this humming noise could be a clogged air filter. Check the air filter and replace if necessary. This can improve your engine's performance and reduce noise.