i hooked up my bluetooth adapter and it works fine except that as soon as i press the telephone to answer there is a very loud thump through the speakers yet the volume is ok i hear the other person perfectly and viseversa.

Try changing the mute option, is yours on All off, -10db or -20db? Maybe there is an amp and you are turning it compleately off.

tried it no effect

Then I have no idea. I guess I would then take out the amp out of the picture and try hooking some car speakers directly to the speaker outs from the Z1, disconnect the speaker hook up you have now and see if it happens from the Z1 or from your car's amp and speakers.

I have speakers directly connected to the Z1 and there is no loud thump. I'm guessing it's whatever is between the Z1 and your speakers, but no knowing your car, I can't guess what that could be. It's also possible that it's a defective Z1.

The Z1 manual does indicate some users will hear noises during answer and hang-ups.

I can understand hang-ups, but i'm not sure I understand on answer.