You’re not alone it seems (Dyson Hot + Cool Purifier clicking on oscillation - Suggestions?). The only choice is to have it repaired or replaced by Dyson. You should be able to claim off their warranty. 

Iv seen this post, looks like he has been on his 4th replacement. 
when you say claim of their warranty how is this possible? 
I purchased this in Argos a year ago now on a normal debit card unfortunately not a credit card :(

spoken to an engineer and he thinks it’s perfectly fine and is normal to make these sounds. I’m currently fighting back as it’s not normal at all. 

Hello @ARKM3 

Thanks for posting a question to the wider community. Sorry to learn about your experience.

Usually when your Dyson switches from front to rear airflow there may be a clicking sound as the motor adjusts direction. I have added a link for you here to guide you on doing a few checks.

When in auto mode it can make different noises, such as an acute high-pitched noise, as it adjusts the airflow to bring the area to the purification target. If continuous monitoring is on, the machine can produce a low hum as it continuous to read the environment’s levels.
By disabling this feature, the machine should stop making this noise. Read the ‘Using Continuous monitoring’ article to learn how to enable and disable this feature. 

If you still experiencing any further issues, and as you’re already in contact with our customer service experts, I’d encourage you to continue your conversation with them. They’re best placed to help you further and are empowered to resolve most issues without the need for further escalation.

If you still need any further help regarding this, don't hesitate to respond over here. 

Regards,

Clicking isn’t usual. I have two Dyson purifiers and they do not click when they oscillate.  

You have a two year cover with Dyson from Purchase. It’s part of their warranty/guarantee service. All you need to do to make a claim is contact them, provide some information (names, address and product serial) and they’ll talk to you about the next steps.

I wish this site had an upvote section! Upvote this! 

Loving all the response I’m getting and to see how many people are having same issues which must go to show it’s actually a known Fault but just not recalled by Dyson. Will have to see how it goes. 
 

Also FYI  @Heradien.5 This has nothing to do with humming or any high pitched noise. This is a clicking sound; sort of like a poor bearing or cog design that rattles away as it oscillates left to right on a low idle speed. Definitely not normal.