So I've lurked for a while and have done a number of small repair jobs thanks to your guys help. This is one I wasn't able to find a thread yet, maybe its that simple

I have a 2000 Dodge Dakota Pickup 3.9. So when I am at low speeds or braked at a light I get this loud rattle from my front end. I had my gf sit in the truck on the brake as I checked where it was coming from and it seems to be from Tensioner as of this diagram here



So what happens is I hear some air release then the tensioner starts rattling against itself. It goes away once I start to accelerate. Any ideas on what I can do? I've yet to touch a belt assembly.

 

Air release? There is nothing that should do that.

Replace the tensioner assembly.

AUTOMATIC BELT TENSIONER 3.9L/5.9L ENGINES REMOVAL
(1) Attach a socket/wrench to mounting bolt of automatic tensioner pulley bolt.
(2) Rotate tensioner assembly clockwise (as viewed from front) until tension has been relieved from belt.
(3) Remove belt from idler pulley first.
(4) Remove belt from other pulleys.
(5) Disconnect wiring and secondary cable from ignition coil.
(6) Remove two mounting bolts and ignition coil from engine.
(7) Remove mounting bolt and tensioner assembly from engine.
(8) Remove pulley bolt. Remove pulley from tensioner.

INSTALLATION
(1) Install pulley and pulley bolt to tensioner. Tighten bolt to 61 N·m (45 ft. lbs.).
(2) Position tensioner assembly and install mounting screw tighten screw to 67 N·m (50 ft. lbs.).
(3) Connect all wiring to ignition coil.
(4) Position coil to tensioner assembly, install two mounting screws. Tighten to 11 N·m (100 in. lbs.)

CAUTION: To prevent damage to coil case, coil mounting bolts must be torqued.

(5) Position drive belt over all pulleys except idler pulley (located between generator and A/C compressor).
(6) Attach a socket/wrench to pulley mounting bolt of automatic tensioner (Fig. 75).
(7) Rotate socket/wrench clockwise. Place belt over idler pulley. Let tensioner rotate back into place. Remove wrench. Be sure belt is properly seated on all pulleys.
(8) Check belt indexing marks.

 

Hi 00dakdan,

Thanks for outlining those steps for me.

I figured I'd throw more pics in since you said nothing should be making that hissing sound.

The hissing type sound just before the rattle is coming from this area here.





The rattling is coming from these two parts knocking into each other. The part with the belt shifts down a bit and then starts hitting the other one.

 

Not sure on the hiss, power steering pump maybe?

Is the belt routed correctly? If so, the tension has probably failed. Maybe you're hearing the belt slipping a bit.