Strange rattle at 2500rpm

[AJRK]

Hi all.. I noticed the last couple of days that my 01 QX4 is making a strange noise while accelerating. I first noticed that my A/C fan seemed louder when accelerating around 2500rpm. Same if it's on ECON or AUTO. So I put down the window and listened, and there seems to be a light rattle coming from the engine area at about 2500rpm. If I don't go over that mark, it runs normal, and if I accelerate beyond it, it's fine too. Any suggestions or ideas of what it might be?
Everything else is in great shape, always well maintained.. only 125,000KM on the clock..

[ARKQX33V6]

Lots of things are possible but noisier A/C fan could be a bad voltage reg and higher voltage to the fan. So check the alt and reg, simply put a DC voltmeter, multimeter across the battery have some one else start the car, note the voltage readings.
Before start with a good battery ...12.5 or higher
While starting...no less than 10 V, but will read 11.5
At cold idle...around 13.5 and up
At warm idle...around 13-14
Over 2500 RPM...at 15 max

Check your clutch fan, main cooling fan for bad clutch, wobble, broken fins.

Check transmission fluid and transfer case fluid

Air box noise

Loose hood holder

Wind up the front windshield over wipers and cowl

With open hood try revving at various speeds and listen

Air valve rattling, cramped EGR, A/C compressor and compressor clutch.......