The engine knocks at higher rpm, only at high rpms

The 3.9L V6 didn't use a knock sensor. The 3.7L V6, used in 2004 and later, had one though.

Sounds like one of the hydraulic lifter may be week dose the sound go away at higher rpms or get louder

5.000rpm's high revs and you run into problems with valve bounce ( hitting pistons) ,ring slap, gudgeon pin knock, big end noise and possible main bearing noise . You may also be getting a hydraulic lock in the hydraulic followers that is holding the valves open too long. Unless the engine has been specifically built for these rpm's any loose ,worn on unsuitable part will cause problems I am thinking that you have around 100.000 miles on the clock after 7 years and the motor is not young Ford V8 engines specifically built for a 500 klm race are limited to 5.500 rpms because they destroy the timing chain if driven over that speed .

The best way to diagnose the noise is to hear it. Without hearing it all anyone can do is guess.
Is it a low dull thud? Goes up as the RPM go up? sound like its low in the motor?
That could be the lower end of the motor going out (worn bearings/rod knock).
Make sure you have oil in it.

Dodge Dakota misses at low rmps but not at higher rpms, what gives?

This trans has a safety built in to default to second gear if the computer sees any kind of problem. Have it scanned. It can be an input or out put speed sensor, or even a temp sensor on the engine. If there are codes that say ratio error, take it back to who rebuilt it and hope they will take care of you. I use to give a major discount if they were not too far out of warranty.

Does the oil light come on or the knock occur during the engine warm up after a cold start?
There could be a couple of problems causing the issue. You could have an oil pressure pump or pump pressure regulator sticking. I would have a mechanical oil pressure gauge installed and verify oil pressure at idle and 2500 rpms to make sure oil pressure is in normal ranges.
You could also have excessive piston rod bearing, crankshaft bearing or cam shaft bearing wear that will cause oil pressure to drop too low to prevent the rod knock your hearing at 2500 rpm.
You said a knock and not a ticking noise,so I don't believe a lifter is the issue at 2500 rpm.
Start with the oil pressure check and go from there.
Let me know.
Regards,

the syncronizers are out on your trans for third gear and as far as the lifters try running some high octane gas with some oil and gas treatment before going into the motor to eliminate that first.

Dear sea_lion42, Friday 2-20-2009 11:22pm EST (USA)

Check your transmission fluid which is usually done with the engine warm and running!
If the tran fluid is low add some Lucas Trans Fluid from your local auto parts store!
The knock makes me believe your low on trans fluid!

jack

Yes they use the same frame.