It's just a fan in a box - either air is getting in where it should not or not enough air is passing through.
You've checked the hose for blockages which is good - now check it for splits etc - where air can enter but should not.
The airflow passes over the motor to cool it down but not before it flows through a filter to keep grit out. Look for the filter and clean (wash) it.
Check the cleaning head at the end of the hose too.
Run the machine without the hose - cover the entry hole with your hand - the motor should speed up as it has no air to move - when you take your hand away you'll hear the motor slow down.
Do the same thing with the hose connected - compare the two sets of sounds - there should be little difference with hose and without hose - if they're different then examine the hose more closely.
Small chance that the motor is worn out and may need new brushes but very unlikely unless you run a cleaning business and use the machine every day - look first to the internal filter and the integrity of the hose.
https://www.youtube.com/watch?v=B71XldoFFfo
Good luck..., January 2015
