Source of engine rattling identified
 

Ever since I replaced the timing belt, water pump and all idler/tensioner pulleys, my engine has been making an odd rattling sound as if the bearings in a pulley were bad, but the sound wasn't quite consistent enough. After taking all the covers back off and running the engine with just the timing belt installed (no fan or acc. belts) I discovered that the timing belt is bouncing up and down between the passenger side cam and water pump pulley, and hitting the small idler which lies on the tensioner pivot bolt, thus making the rattling noise. Has anyone had a similar problem? It is much more noticable around 1500 rpm. As far as I can tell the timing belt is tensioned properly (I have the aftermarket tool).

sounds like worn teeth on cam pulley / main pulley. I would check all the teeth on both cams, on the crank, and the one on the oil pump.....but I really do think you have worn teeth on at least one and more likely on more than one....

IIRC one or both cam gears had some minor wear where the finish had worn off several teeth. Is that enough to cause this? Pretty sure the crank and oil pump were fine.

BTW is the timing belt ever supposed to contact that particular idler, ex. when it is cold or when up to temp?

That is exactly what that idler is for, to keep the belt from bouncing too much. I think you need to check belt tension again. (when cold of course)

Alright, I know I checked the belt tension about 1K miles after the service and it was loose so I snugged it up (using tension tool of course). But since then I have gone another 2k miles, so I will check it in the morning.

BTW does the oil-filled timing belt tensioner slacken or tighten the belt as the engine heats up?

the tensioner releases tension as it gets hotter.
What kind of oil is in the tensioner,
what kind of pump did you install ,
what kind of belt did you use.
What was the condition of the idler rollers??
Do you see any marks on the belt ,
has the belt gotten oil or coolant spilled on it??

Did you install new plastic bushings in the idler arm during your timing belt job?

Yes, everything that should be replaced during TB service was replaced

80-W90 gear oil in tensioner
new plastic bushings
all new idler rollers
new Laso water pump
new belt (of course), Contitech I believe
No fluid has contaminated belt, I replaced all cam seals and cam housing gaskets as well + t-stat and water bridge gaskets
what sort of marks am I looking for on the belt? Perhaps I will just take a picture of it