I get a sound from my crank as if something were about to break to pieces when I stand and pedal hard.
You should have this checked by a mechanic immediately. There are at least 4 possibilities:
- One of the crank arms is loose or cracked. This is the most dangerous, since a failure at the wrong time could cause you to lose control of the bike.
- A bearing is loose or worn out in either the bottom bracket or one of your pedals. A pedal could also fail catastrophically, though it's unusual.
- The chainring bolts might be loose.
- The bike's frame might be cracked. Unlikely, and worth checking only after other possibilities have been eliminated.
The most common cause tends to be a worn out bottom bracket bearing or loose crank arm.