Grinding can occur in all BB system types and can be the result of many factors including the lack of grease in the unit due to dirt and/or water ingress or worn parts in the BB. Alternatively the adjusting cup can be too tight in an open/loose unit resulting in the grinding sound.
WORKSHOP TIP: There are times when you will only be able to hear any noise coming out of the BB when the bike is actually being ridden – sometimes quite hard. At other times you will be able to turn the cranks or the BB spindle and hear the grinding easily.
If the BB is grinding, remove the chain from the chainring and rotate the crank arms.
If an open/loose (adjustable) BB feels dry or gritty and the turning motion is uneven in places, you can try adjusting it to see if this improves matters (although the BB can still need re-greasing). if adjusting does not help then it is time for an overhaul.
Grinding in a sealed BB is a sign that it is deteriorating and it will need to be replaced.
Click here for more information on fixing BB issues.