My guess rod bearings.
Inspect the oil for metal shavings.
You can take the oil pan off and physically wiggle the rod caps, they should all be tight with about a milimeter of play side to side, or less. if one is totally out of wack then you need to do a rebuild.
You could try to replace just the rod bearings, you can do that in some cars without pulling the engine, my buddy just did his rod bearings in his VW with ease and it fixed his "clunking" problem....I say clunking because "knock" means something totally different on this forum.
The Neon motor is known for its piston slap, its a POS.
If your bearings are going bad then you should try to fix it as soon as possible.
I hear that you can build up a 420a and make some nice power with forged internals
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