hissing sound with rf modulator and power off

I just installed a Scosche rf modulator from Crutchfield and it works fine with one problem - when I remove the iPod and shut off the radio and turn off the car and remove the key I hear a faint hissing sound coming out of the speakers. I went so far as to remove the power to the modulator (i.e., pulled the power connection from the fuse box) and I still get the hissing sound. It shut off after about ten minutes. Any one else run into this problem? Is there some sort of power bleed through the radio back through the antenna?

Does it happen after you turn the Scoche unit off as well?

It happens even with the unit disconnected from power. After cousulting with a stereo shop and a Porsche dealership, I dismantled the whole thing. The stock stereo, hooked up as from the factory still does the same thing. Has anyone else noticed this? The stereo guy said this is probably normal for cars that don't shut all systems off as soon as you turn off the key. The guy from the dealership had no idea but said I might have damaged my radio (which I can't imagine given that I just unplugged the antenna). I reinstalled the modulator and am going to try it out for a while. I should point out that the radio makes a clicking sound and the hissing stops exactly 5 minutes after the key is turned to the off position and removed.

It sounds like a ground issue. I'm not sure where you would get a short from adding a RF mod but I'd look into that pretty close. Suneet knows infinitly more about this then I do but the noise is ending when some relays time out after you take the key out.

Well, the slight hiss continued to occur regularly even after I had entirely disconnected the modulator and wiring. I'd be curious to see if someone else duplicates this with a stock set up. If not, then it might be an indication that I have somehow damaged my radio. If I open the driver's door after the car has been sitting for a while, I can hear it. You have to have your ear up against the small door speaker and little background noise, but it's noticeable.

Bose or Sound Package Plus? If SPP (and perhaps Bose, but not sure - I expect it to be the same), the amp is on a separate circuit and may be timing out longer than the head unit. As is the case with many, many (dare I say most?) amps there is a bit of hiss at high volume settings which is simply analog noise in the system being amplified. It's there as long as power is applied to the amp. With the system on but no audio source (CD stopped? I'd suggest using the TP button which serves to mute the audio but I don't know just how and where in the system that's wired in.) can you change the volume of the hiss by changing the volume setting on your head unit? If so, that's probably it.

I'm pretty sure its regular on sound package plus cars. I highly doubt you damaged your radio with an FM modulator install. Also, if it was a ground issue you'd have a hiss or a hum throughout listening to the iPod. I remember a speaker hiss on my previous Cayman with SPP... I don't have it in the new one with Bose.

I have the Bose setup. I'm wondering if it's been there all the time and I never noticed it before (I was in the car with the radio and all power off; to hear it with the doors open and any ambient noise you have to have your ear right up against the speaker). I'm pretty sure there's no radio damage either; it's not like I messed with the radio power or spliced into the radio harness anywhere. With the radio on and iPod hooked up I notice some of the typical noise and hiss associated with that setup. Turning up the sound turns the hiss up too. However, the hiss with power off is not affected by the volume knob.

Mostlike there all along, but you just noticed it now. Especially at the low levels you noted. IN this respect,t he Bose is like any other amplifier - it has inherent analog noise, and sometimes, that will just plain show up as a very low level hiss. A hum would be different - that would indicate a ground loop or other feedback in the circuit that remains in the low frequency band. Doesn't sound like it unless you unplugged/modified other wires or connections. Worst you could do to yourself in unplugging the antenna cable is to break something in the cable or the connector and completely lose radio (and RF modulator) reception. I would not be surprised at some hiss (but at a really low level). If you had some hum then I'd suspect a faulty ground connection, but as I noted earlier, only if you did anything with connections and or wiring other than the antenna cable. Seems right - with power off, if the amp is staying on longer (and it seems that it is) but the head unit is powering down, then the volume control should have no effect. That's why I asked if you could affect the volume level of the hiss with the power on but no audio source (such as with CD selected but stopped).

I'll check the next time I get in my car to see if I can hear anything... The funny thing is even when you power the headunit off or take out the key, it is still active. The MOST chain stays on for around another two minutes, even after you lock the car and are gone. If I turn everything off and take the key out, I can still hear the headunit sometimes spinning a cd, and it will always eject and turn on with the key in or out.

Well, as Pharaoh might have done in a similar situation, I decree that my radio is as it should be and this power-off hissing was there all along. As for the R/F modulator installation, it works as advertised as well. Sound quality is ok, but there is no fading in and out of the signal or interference from FM stations like I had with the wireless model. But for $40, it is a very cheap alternative to any of the others that I've seen so far. And many thanks for the VERY helpful radio removal instructions on the site and also for the help with diagnosing this issue (which is no longer an issue, so say I).

Cool. Just think you just saved hundreds of dollars and got the same solution/sound quality as the official Porsche adapter. That has to make you feel warm inside.

I noticed the same thing on my CS with Bose. There's a very faint background hiss like the amp is still powered. The car is new and the sound system untouched.

Yes, it does make me feel very good to know I saved a bunch of money going this route. I would love to have something that has a little better sound, but I just can't bring myself to spend more modifying the car than the iPod itself cost! I appreciate the help in confirming that others notice the same behavior from their stereos/amps/speakers.