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Hi and welcome. When did you last clean the filters? To be honest it doesn't sound different to any other DC25. When you pull the wand up dose the change over valve operate or is it sticking?
If you put the crevice tool on and block the suction with your hand does the suction relief valve hiss or does the suction sound like its leaking from the changeover valve? My DC25 Animal did that, it was a 2011 model. Also, I suggest taking the wand and hose off the vacuum and see if it makes the noise. If the problem persists, then it's somewhere in the machine
You're welcome :) PS I think it's the post motor filter seal (filter in the ball) as it is a very common issue that affected all DC25's. The seal would start howling especially whent he machine was turned off, and often when the machine was turned on and it would often continue aswell. Check that the seal is fitted firmly.
Most likely the post motor filter, try running a bit of silicone grease over the rubber and re fitting it.