Oh spanners!

[405GLX]

I have just came back from a drive in the 405 1.9TD, and during the drive I got stuck behind a Nissan Micra traveling at 50MPH in a 60 zone, so I shifted down a gear and floored it, but once the needle passed 4,500RPM the engine started thumping!

Once I'd finished overtaking I immediately changed back into 5th and the revs dropped back down again, and the thump stopped.

So basically... What is wrong with the engine!?

Here are the details about the engine to help you help me (hopefully)

1905CC XUD Indirect-Injection Turbocharged Diesel 8-Valve
299,000 Miles on the clock
Pistons replaced around 2,000 miles ago
Oil changed about 100 miles ago with Mobil Super 2000 15W40, and Comma 75,000 Plus Oil Additive added.

Another thing I thought of, was the clutch/flywheel, and it has the original clutch/flywheel.

So... Can you guys help!?