Ok might just be worth checking there isnt anything stuck in the fan at the front.
To check this you need to remove the filter in the front as you open the door and look down there and see if you can see anything in there.
Then make sure the dryer is unplugged
Then take the condenser chamber out on the right hand side.
Then just put you hand in there on the left at the front and you will be able to reach in there and feel the fan.
Check to see if you can feel anything in there.
If not then its possible the front felt bearing unit may be on the way out or the rear bearing as to be honest we dont replace many motor for being noisy on these.
So if thats the case then if you are getting some one in to repair then you are looking at about the £70 mark roughly.
So its a gamble as to weather to repair or replace.
But check that fan first as you may just find something has got in there and thats causeing the noise.