Volvo: vibrating under car/electrical problems, morgan morgan, battery indicator

Question
Hi there:

I hope you can help with a few questions I have regarding my 1990 740 Turbo Intercooler wagon.  It has 129,000 miles on it.

First, the battery indicator drops drastically when I turn on the air conditioner and only somewhat when I turn on the lights.   My usual mechanic has run tests on the battery and it seems fine but he can't explain why the indicator goes down when various things are turned on.

Second, there is a vibrating from underneath the car at the back that I notice (varying in strength) when I'm stopped at a light or stop sign.  The muffler visibly vibrates when I have it in park and check it.   Could this be something with the exhaust system?

Any assistance with these questions will be greatly appreciated.
Thanks in advance,

Morgan  

Answer
Morgan,
Has the charging system been tested? You may want to pull out the built in regulator and see if the brushes are worn out. Maybe the A/C compressor is going out and drawing to much power. Have the amp draw tested.
The exhaust system has hangers are they intact? If not that might be you vibration. Have someone sit with the brake on in the car and then look to see if you can tell where all the vibration is coming from.