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I have a Fortwo Cabrio Passion, 2008. It has been in storage for three years but I got it back about a year ago.
I had it checked out twice by dealerships and it was given a clean bill of health.
However, lately there has been a really strange noise. When turning it over there was what I can only describe as a deep base sound, almost like a reverberation in a speaker. It happened randomly and when I could not make it happen (it continued but randomly, seemed to be no set pattern to it) I decided it would not be worth taking it to a shop. Figured it would not happen when I was at the shop and they would think I was nuts.
Then, today, on the way home the noise came back. This time while I was driving. The radio is never used, I listen to audio books on my blu tooth ear plugs, but this was really loud. It scared me so I pulled over. It stopped. I turned the car off, back on and got back on the road. It came back but then went away. I was only about 3 miles from home so I went on home and parked the car. I turned it off and left it for a bit. Turned it back on and I get the loud thump when I turn the key and the ignition starts, but it goes away.
The car seems to run fine, no vibrations, no other issues. The brakes were recently done (after sitting for three years they needed a complete change out), back tires replaced and some other maintenance but nothing I can connect to this.

Any ideas? I am open to all suggestions.
 

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So what is the cause of this "farting" noise coming from the sub woofer? I have replaced the capacitors in the sub to no avail. I noticed I can make it "fart" by pressing on the up or down window buttons when the windows are already all the way up or down. I can also make it happen if I step on the brake pedal at a stop sign... really any time I put a load on the electrical system at idle. I'm beginning to think it might be a low voltage situation. Perhaps the voltage at the sub amp is low due to a bad positive or ground? Does anyone know the pin outs at the connector to the sub? I have a feeling a lot of people have encountered this situation.
 

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Couple of years ago I unplugged my subwoofer because of the noise is it was making. I'm going to check out the low voltage theory. The beginning of this year I purchased a new battery Duracell absorbent glass mat battery from Sam's Club. Today I plugged my subwoofer back in and we'll see if it starts making the weird noises. If it doesn't make weird noises then I think we have found a low voltage warning signal for a battery that's going bad.
 

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MY08 cabrio MY09 Brabus MY21 Bolt
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I was shocked to find that a battery (AGM) change may have fixed my growling sub!

A couple weeks later, all is good…

 

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I was shocked to find that a battery (AGM) change may have fixed my growling sub!

A couple weeks later, all is good…

Sadly not here. As I stated in above post I got a new battery at the beginning of the year and just yesterday plug my subwoofer back in. Last night while taking Evelyn up to the park, when I turned off the car the burping or farting sound happened then again at home.
Today same thing turn the car off at the park and It made the sound, turn the car off in the garage same thing. I went ahead and unplugged it again.
 

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From Las Vegas NV
Welcome to SCoA forums
You Do Not have to remove any seats to unplug the subwoofer on a U.S. spec 451.
U.K. smarts may be different.
Take a look at the picture in Post #18 at this link
Please Disregard the flood title and how dirty the floor was in my car
 
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