It'll likely be the speed controller. Part number TRA027
Mine's been back for a warranty repair and I don't envy you.
From memory it's Toolstream that deals with all Triton warranty repairs and by the looks of it they employ toddlers to do the job. Mine came back with the top housing dislodged as they routed the wires for the speed controller wrong. All the black screws were marked like they used a pozi bit in the philips heads. ( I keep my tools in pristine condition and look after them)
Complained about this (the router was only a few month old) and they agreed to take it back and supply me with a new one. Sent it off as a bare unit and a few days later I received a brand new sealed box with all attachments, collets, handle etc so have some spares now.
Even if it's under warranty their policy states the end user has to pay for shipping. Speak to the manager and she'll waver that. I'll try and find her name in one of my emails.
Good luck with it. If it's still under warranty just get them to sort it and keep your fingers crossed. If out of warranty just cough up for the 50 quid speed controller and fit it yourself. It's a 10 minute job including a coffee break.
Mine has been no bother and has been hanging in my router table for about 3 years iirc.
