got strange sound from under truck?????

Hello guys, I got a sound coming from under truck, it’s like a rattling noise not real loud last about 3-6 seconds at different RPM's.  It has no rhythm or consistency.  it will do it at idle when I raise rpm to about 2000, then u let off or give it more it stops and same when driving, when it starts I can give it more gas or let off and it stops.  I can't find anything loose that could make it. No vibration occurs with it. sounds like a small screw going through a fan. The only other noise that I get is a small vibration between 40-60 mph like a tire noise, but I got new tires and drive shaft looks good.
just got engine and xmission rebuilt, they sound good and can't here any noise from them, like the sound i here under truck.  anybody got any ideas for me to check??? thanks  

Hmmm that could be alot of things... Try playing with your fan...it could be worn down... Or it could possibly be your water pump is going bad...does the temp stay pretty much consistent or does it rise and lower at random points... I suggest these two problems because well they are two common problems and two easily fixed problems  

got electric fans, pump is new, the noise is concentrated under and at right side muffler, its strange man  

Could be that your muffler is lose.My truck did the same thing after I put a glass pack on.Could also be the dust sheild on you rear drive shaft where it goes inthe tranny.I was hit by a pipe and it knocked that sheild off and it maybe a hell of a noise be fore I put a muffler clamp on it.But thats move of a sreech.I'd look at the pipes to see it there's any movement in them.  
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hey cool thanks, still cant find it, everything is tight, it is crazy man!!!!!
u know where i could get a cheap xmission for a  90 pathfider 4x4.
thanks for ur help, L88ter  

does the truck have a converter on it? i've seen the cores in stock converters come loose and make a hellicious rattle, and even sound like an engine knock from time to time, sometimes they quit after warm up others don't. also check your bell housing bolts, if one side has come loose it'll make a hell of a rattle that you can't notice just but looking at it, also if its auto remove the 6 15mm bolts that hold the torque converter cover drop it down and make sure there are no loose torque converter bolts and that the flexplate doesn't have any cracks  

yes and its new, checked the bolts its all good and no noise from the converter or flex plate.  the only place i hear the noise is muffler, but when i try move the muff or bang on it with truck off no noise, so again im lost in the search 4 the crazy noise.  thanks  

what kind of muffler is it? if you here the noise coming from the area of the muffler its most likely something inside the muffler is loose but you probably don't hear it without the force of air going through it.  

yea i didnt think about it that way, it is a hooker muffler.  

check catalitic converter heat shield as well.  

no catalitic converters on truck, but i think i have found it its the muffler and the sound travels up to headers.  the muffs hangers broke a few months ago and i had them zip tied up until i could get them welded, so i think during that time the banging against crossmembers and frame caused a chamber wall to come loose.  so now i gotta get new ones.  thanks for reply l88ter  

Well I have a 1997 Ford Ranger and mine done the same thing and it was my cats so check that.....Let me know wut u find....  

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