Hey all...after smelling slight coolant smell (but no visible leaks) after warming up and driving (especially in enclosed areas, ie; garage), I thought I would start by replacing orig. radiator cap and go from there. After replacing cap and letting engine idle until hot, I noticed a slight intermittant knocking type noise at idle. If I blipped the throttle past idle to any rpm, the noise dissapears, return to hot idle, noise returns. Any ideas?...noise is not major, but is noticeable.

Sounds like your UIM has failed, check your oil.

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-Car Guy Carl
"Penelope" - 2003 SLE - 247k Miles
"Sydney" - 2000 Honda S2000 - 98k miles

Thanks Carl...oil level and condition seems perfect, no kind of "milk" under cap, doesn't smell "out of the ordinary". One thing I have noticed after replacing rad cap is that the coolant in radiator seems much darker that coolant in res tank (almost pink hue in tank), radiator coolant seems to have a slight brownish tint to it. I was under the impression that the color diff. is normal.

Any thoughts?.

I think you have a failed LIM at the very least and possibly the EGR tube in the plenum. - BC

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In Memory of Brad - 1/21/1977 .. 10/23/2013 ...... Aaron - 1977 .. 2017 .....
2017 Hyundai Santa Fe Sport ..... 1992 Bonneville SSE 1SB 170 HP L27 4T60E retired/sold to MattStrike ..... 2005 Bonneville SE 1SC 205 HP L36 4T65E - retired/salvage yard ..... PBCF user 2321

Would you be able to get a video from us, can you pinpoint the whereabout of the sound at all?

_________________
-Car Guy Carl
"Penelope" - 2003 SLE - 247k Miles
"Sydney" - 2000 Honda S2000 - 98k miles

I've also heard that a failed/worn harmonic balancer could cause this type of interm. knocking noise when hot...
Any thoughts?.

A failed balancer will knock at any temperature at idle and usually goes away just off idle. The knock sound they make is horrible, would surely make you think the engine is blowing up.

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Ryan

2003 Bonneville SSEi - The Black Mirror SOLD!
2002 Bonneville SE - The Mutt Complete 2004 SLE interior, drivetrain, and body harness swap, ECC swap, HUD swap, black GXP wheels, GXP headlights and tinted tails - SOLD
2003 BMW 540i M Sport, 2001 BMW X5 4.4i, 2010 GMC Acadia, 2017 Grand Design Imagine 3150BH
1982 Cutlass Supreme - The fun one

Hooray!!...finally found the cause of my knock/clatter noise when hot. During leaking oil pan gasket replacement yesterday, my mechanic checked the rod bearings when he had the pan off and sure 'nuff, the bearings were absolutley showing wear patterns/slight streaking. Good news is that the crank itself is primo, no problems...looks like we caught it just in time before nightmare damage. Replaced rod bearing set and pan gasket, filled up with Mobil 1 hm syn., installed Mobil 1 filter and now she purrs like a kitty!.

Thanks for posting the follow up.
Always good to know what the actual causes/fixes are.