V6 engine knocking and squeaking noise

There is nothing in the engine that remains at a constant speed as revs increase. My bet is the climate control system, circulation fan or a blend door actuator. They would be the other side of the firewall whilst you are in the engine bay and the sound is behind the engine.

The other thing you can try is a piece of plastic pipe in your ear and use the other end as a stethoscope.

That depends on what kind of knock, but if it is emminating from a bearing or a piston you will need to rebuild the engine.

Have you checked the oil pressure.

Remove the engine drive belt and run to eliminate any accessories causing noise.

Check valve rockers, could have a bad hydraulic cam follower that is not pumping up.

Could be more serious, why did the head gasket fail, might be an associated problem with that if engine overheated ect.

It sounds rather like one of the drive shafts may have broken, but that is rare. What happens when you rev the engine in neutral, any knocking noises then? Does the oil pressure warning light come on? Is the check engine light on?

I'm trying to deduce whether the noise is in the engine or transmission somewhere.

The only cure to fix squeaking belts, is to replace with the new one. When the belts start squeaking noise, then it is the time to check the status of the timing belt or chain, depending on the size of the engine.
The 2000-2004 Nissan Xterra was offered with the choice of 2.4L 4-cylinder or 3.3L V6 engine.
The 2.4-liter 4-cylinder DOHC engine was a pretty good reliable engine, but for this truck, it was underpowered. This engine has a long-lasting timing chain - there is no timing belt to replace.
The 3.3-liter V6 SOHC was also a bit slow on pick-up, but for daily use, it was pretty good, durable, simple engine. This engine has a timing belt that needs to be replaced at recommended intervals. The SE model offered a supercharged version of the same engine with more power.
To get more information, please read my tip :
When should I change the timing belt on my car?
http://www.fixya.com/cars/r6481199-when_change_timing_belt_car

Change the belt

Sounds like you didn't put the pushrods back in the proper locations. There are two different lengths in this engine and you may have one backwards

hey i dont know if u already solved ur problem but i was having the same problem with mine so i went and bought this solution called risoline, compresion repair. when i changed my oil i poured that stuff in first and then the oil. with in a day it stoped the clicking/knocking noise. hope this helps

that sounds like the squeler fo the brakes and you might want to check to see if they need to be changed and/or have you had them changed yet?

big end or main bearing.