3 mysterious noises
 

My '99 base Boxster makes 3 noises that I'd like to know more about. The car has 54,000 miles and is in great shape and in good tune. I don't think any of these noises are problematic or abnormal, I just wonder if anybody can tell me what they're about.

Noise #1: When I start the car when it's stone cold, for the first half second there's a noticeable clatter from the engine, which goes away right away. Blink and you miss it, but it *is* distinct. Is this metal-on-metal, the pistons hitting the sides of the cylinders, before oil pressure has built up and there is some lube there? Totally normal and unavoidable, right? (Fingers crossed.) This is before and after recent Mobil One oil change.

Noise #2: Upon starting the car cold, I hear an electric motor over my right shoulder, and it runs for the first full minute or 90 seconds, and then you can hear it wind down in a pretty distinct way. Before the car is fully warmed up, that motor will cycle on again once or twice, for just about 5 seconds each time. It sounds cool, I don't mind it, it's another part of the symphony, I just have no idea what it is.

Noise #3: When I'm at a dead stop and I'm letting the clutch out to take off, and not giving it enough gas, if the RPMs fall below about 1,000, over my left shoulder the car gives a pretty significant sigh. It sounds like air being forced out of a bellows. Something to do with the clutch? Any ideas?

The bonus noise is my favorite, that I think even I can guess at. When I pull into the garage (little bit of an echo chamber) and turn off the car, there is a hum that resonates deeply and takes a full 5 seconds before it fades away to nothing. It's my consolation prize for a fun drive coming to an end. I assume it's some part of the exhaust system just settling down for a nap. Well, maybe you know more of the specifics- I realize cars don't really nap. Or maybe they do.

Any light you can shine on these things is appreciated.

Thanks.

All perfectly normal. As for number 3 don't do that to your engine or you will start to have problems.

I have only witnessed the 1st sound once. Also for the second noise, are you sure it's not the Fan? I can also hear that electrical sound, it but it started doing it after I changed my AOS. Maybe something related? I'm sure it's not the fan.

As for the bonus sound, it only happens to me if I rev up a little aNd then shut down the engine ( before the revs calm down)

1st is likely lifters pumping up.

2nd is your secondary air injection.

Third sounds weird, not sure what that is, but i doubt its harmful

So what shall we do about the SAI? Replace it and if so, how much ?

The SAI pump should only run aprox. 90 seconds on cold start up. Its a distinct sound different from the engine compartment fan.
The engine compartment fan should only come on with much heat(don't know the degree value) within the engine bay. You will feel the air flowing through the right side vent for the fan but not the SAI.
Not sure what tells the SAI to come on. I know If I crank the engine on a cold start but turn it off before it starts, my SAI will then not come on for that cold start

You certainly don't need to replace if it runs for the initial 90 seconds. They are expensive

Thanks. Yes it runs for almost a minute or so. But it's weird cause only until recently it was not making any sounds. After I replaced my AOS, it started whining

Engine Compartment Purge Fan
To reduce the temperature of the engine compartment, a fan actuated by the DME is installed behind the ventilation grille of the passenger side rear fender vent.

If the fan is turned on, fresh air is fed into the engine compartment. There is a temperature sensor mounted in the engine compartment to provide temperature input to the DME.

Standard Operation:
1. With engine running or ignition on:
If the engine temperature is higher than 102° C and the engine compartment temperature is higher than 60°C, the engine compartment fan is switched on for 30 seconds by the DME control unit and the engine compartment is thus ventilated.
2. With the engine off and vehicle is stationary:
If the engine compartment temperature is higher than 60° C when the engine is switched off (ignition off) or if the engine stalls (engine speed: 0 rpm), the engine compartment fan is switched on for 20 mins.

Note: If the engine temperature has not dropped below 80.25°C after the engine compartment fan has been running for 25 seconds, a fan fault is registered and the coolant level warning lamp is activated (flashes).

The secondary air pump will turn on under two situations: (1) At cold start for 90 sec to reduce emissions, and (2) to test the pressurization integrity of the SAI and fuel system.

The pressurization test usually happens the first time that the car is at idle as the engine warms up. However, this test can be delayed substantially if the car never has an opportunity to idle long enough for the test to complete. For example, the car is started and driven directly down the street and stops at a light, but the light turns green and you drive away before the test can complete.

There is no absolute need to replace the SAI pump unless you can't stand the whine or if the pump fails and you need it working to pass a smog test (and can live with a CEL). The engine might run a little rough during cold start but nothing else bad will happen (except you'll be polluting more at startup).

You can find an affordable used SAI pump from Woody or eBay (here is one for $50: 996/986 SAI Pump). They are easy to replace yourself.

Thanks, everybody! Good stuff. The SAI noise doesn't bother me at all/wouldn't change it if I could. Clutch wheeze is still intriguing, but That986's comments just gives me an excuse to rev it more. thstone, thanks for the detailed breakdown.