Odd "hissing", crackling, "depressurizing" sound after highway driving

[Dimitrios]

This has been going for about the last 2 weeks, but intermittently so - happened today after I parked at work.<O:p</O:p
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After shutting the engine off after highway driving (say 75-80 MPH), but not necessarily always (even cruising at 40-45 MPH), I’ve been getting an odd metallic crackling/hissing sound from the front end. Popping the hood and putting an ear close, it sounds like near/in/around the radiator. No dried coolant anywhere or puddles underneath. I don’t see any leaks, bubbles, etc.. Cap is on tight/proper. Fluids are fine. Temp stays fine, never goes above where it should be (just below the mid-point).<O:p</O:p

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It lasts for about 7-10 seconds (if that) then dissipates. Again, sounds like something is depressurizing, but the crackling is boggling me.<O:p</O:p

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It could very well be things expanding/contracting after the drive (so ambient air temperature these past few weeks may be an issue. This is the first winter I’ve driven w/o the heat shield on the stock manifold. Not sure if the additional heat vs. greater temperature differential (than say summer/fall temps) is contributing to this. Possibly other than speed and duration of the drive, I can’t find any common thread so far.<O:p</O:p

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I’ll try to replicate at lower speeds, less duration of driving.<O:p</O:p

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Any thoughts would be appreciated.<O:p</O:p

D

[RyanJayG]

more than likely its just expanding contracting metal

[Protephile]

^^what he said. My car always crackles a bit after a long drive. I only notice it inside my garage, though.

[pasadena_commut]

Fluids are fine.

Don't just check the overflow reservoir. Wait until your car is cold and then open the radiator cap and verify that the fluid is where it should be. My old 323 twice developed pin holes in the pipe that ran to the radiator. Once it made a mess and was obvious. The other time the only symptom was an occasional bubbling sound behind the dashboard. In both cases the overflow reservoir was filled properly but the radiator was low. It never overheated, although it probably would have eventually the time it spit fluid all over the driveway.

[i12drivemyMP5]

Mine does that all the time since replacing the air intake. Are you sure it's not just the compression bleeding off after turning off the motor? I don't really notice it unless in the garage either-harder to hear outside in the open.

[Dimitrios]

Don't just check the overflow reservoir. Wait until your car is cold and then open the radiator cap and verify that the fluid is where it should be. My old 323 twice developed pin holes in the pipe that ran to the radiator. Once it made a mess and was obvious. The other time the only symptom was an occasional bubbling sound behind the dashboard. In both cases the overflow reservoir was filled properly but the radiator was low. It never overheated, although it probably would have eventually the time it spit fluid all over the driveway.

I'll check the radiator fluid; to be honest, I didn't check that.

[RicesP5]

heyy mine does that wierd thing too .. me and all my buddies just joke " oh its my radiator blow off valve.." but latly its started to bother me and i would like to know too .. it just sounds lke the radiator is shooting off air or something ..

[Dimitrios]

^ it's like a big bowl of metallic Rice Krispies with green coolant milk.....I'll have to keep tabs on it...good to know (sorta) that others have the same/similar problem...

[Jim Frye]

Our '03 P5 does it after a drive on the Eway. No hissing, per se, but a bunch of metalic crackling and creaking. It goes away after a cool down period. When the car was new in '03, I crawled underneath and it was the exhaust system cooling down and contracting. The car only has 13K miles on it. My '97 Civic did the same thing when it was new and again when I put a new exhaust system on it. As long as there is no fluids loss from anything, I'd not worry about the noise.

[glyph]

It is common when you stop and get out to hear the sounds of the metal cooling down again.

[RaiderMP5]

It is normal to hear those sounds.