Got a new brushless motor.i fixed it on slow fly plane.i flew the model but while landing about 7metres from ground the propeller came off from the motor .the prop was in the air but my plane crashed .the ground was sandy.some sand went inside the motor.first the motor was running smoothly.now it is making noises.please help to resolve this problem!

clean it see if shaft is bent the magnets may be touching the stator

this is  a common problem post crash.

buddy if it contain bearing.it is damaged.otherwise check for bend shafts

Shaft is not bent

Is it a pure sand, something like in the river bed or seashore?
Then chances are that magnet should have attracted the iron dust and thus the sound. inspect it clearly under bright light.

Ashok.P

Check for what V2 eagle has replied.

Same way some time the outrunner might touch the motor mount this lead to noise better remove motor from the mount and check.

I noticed carefully the sand particles are stuck between magnets and bearings.  I checked everything the outrunner is not touching the mount.