The last time I had a creak on my bike it was down to the pedal crank. It started off with a creaking when pedalling, it soon turned into a periodic creak and clicking with movement and then one night the entire left crank fell off half way home on my 5 mile commute
I had to nurse to the local bike shop, about a mile away, and kept having to kick the crank back on every couple of yards!!!
Moral of the story is, don't buy a cheap MTB from a catalogue and expect it to work after more than a year!!!.