Triumph have been running a programme to make changes to the engine on Explorers up to VIN numbers 602960, yours is 599051 and so would be covered under this programme. Triumph have been offering this modification via post and you should have received a letter.
Here is an extract from the letter sent to affected Explorer owners;
You may be aware that we have made an improvement to the Explorer / Trophy engine – notably to remove a top end ticking that can appear at certain engine revs. This ticking generally becomes more prominent as your motorcycle approaches 10,000 miles.
The noise comes from excessive play from within the valve train. We know that some of our customers have been irritated by the unwelcome noise and we have made modifications to these bikes. We would therefore like to offer you a free of charge engine upgrade here at the Triumph factory to ensure you are never troubled by this symptom.
If you would like to take advantage of this offer just let me know and we will make arraignments for your Triumph to be sent to the factory. Please be aware that Triumph are booked up until approximately March. We will then send you a confirmation email with all the dates and details once we have them.