That's nothing to worry about. The whooping noise is air coming between the post motor filter and it's seals. This is very common on DC25, DC40, DC41 and DC55 machines.
Yours doesn't sound too bad, they can be much, much louder.
If you want to try and get rid of it, give the post motor filter a good clean. Then clean the two black seals where the filter sits. Dirt can get trapped within the seals, but be careful to not break the seals when getting in there. A vehicle rubber cleaner or restorer is good to use for this job. Once clean, bend and manipulate the seals upwards so they're not as flat. Fingers crossed (unless your post motor filter is too worn) this should work.