If the noise sounds like a dull thud or knock from deep within the engine, then I have bad news. Something inside the engine is probably trying to get out. And if it does, the engine will be instantly turned to junk. The worse news is that it probably already is (junk) even though it’s still running, because the damage has already been done. From the sound of things, your engine is perhaps only a few seconds away from exploding expensively.
Noises like this are usually caused by wear in the bearings – typically the con-rod bearings – and while they can be fixed if you do so before the engine grenades, it will always be an expensive job. In fact, it would likely cost more than the actual value of a 2005 Pulsar itself.
The best advice, then, is to let a mechanic have a listen to see if it’s as bad as you’ve described. From there, I reckon a second-hand engine form a wrecked Pulsar would be your best bet if you’re determined to keep the car.