'Backfiring' is when a car makes a loud "bang" (sudden, short, loud noise). I think it's a result of exhaust gases going back into the cylinder instead of out the valve or something like that.

Yes, Sven is correct about the meaning of 'to startle'. Hier ist ein ander Beispeil: "I was really startled when that car back-fired." Cars don't really do that anymore but, well, you know what I mean.

The frightening noise scared me; this caused me to become startled.

Frighten is the same as scare. Startle is a little different.... startle is a result of something sudden. When something happens that you dont expect...... maybe someone screams behind you..... you startle. Something can frighten/scare you (walking in the dark) without you being startled. If you are scared of the dark and a cat falls on your head, it startles you although you were not frightened of the cat. I hope that helps.

Hi Sven, Thank you very much indeed for your help. It is very much appreciated!!