The KLX 250 is notorious for cam chain tensioner failure; it is not uncommon to have the tensioner fail after only 8000 km/5000 mi.
In fact, I purchased my personal KLX in 2017 with ~8,300 km with a failed tensioner. The bike sounded OK at idle but anything past a certain engine speed would make a loud buzzing noise. The previous owner diagnosed the issue as a failed tensioner and had purchased a Krieger brand manual cam chain tensioner, but decided to sell the bike before installing it.
Luckily the diagnosis was correct, and when I installed the cam chain tensioner the noise was completely gone and the motor sounded normal. Depending on your engine mileage, a manual cam chain tensioners might require periodic adjustment- you will hear a slight rattle when the engine is up to temperature if adjustment is needed. I’ve needed to readjust twice since 2017 and the cam chain has been quiet ever since.
I’ve had great experiences with the Krieger cam chain tensioner, and it has my recommendation: