I cannot believe that Q is asking for BMW advice. Even stranger, I cannot believe I'm in a position to answer, at least in part.

I have the 2nd to 1st gear bang as well. What's been beating me to death is that the issue only happens in regular drive mode, NEVER in S mode. Also, issue is in the least dependent on engine idle speed and/or trans fluid temperature.

Interestingly enough another member on Xoutpost recently updated that he deleted his factory water cooled trans line setup for an exterior mounted air cooled trans line setup and his woes are gone. Issue with that is depending on ambient temperature where one lives, the trans fluid may not warm up as it should when it's very cold outside. Unless you live in Quatar, the higher band of the ambient temperature scale should be fine with the external setup though.

That said, I'm going to do a drain and fill with factory fluid first and foremost. Simple procedure to do. Very easy to access the drain and fill plugs on the E53 4.6/4.4(same trans). If this does solve my issue, I'll post up and inform the crew. In the least I'm hoping for an abatement of this darned issue.

My "banging" also does not occur when the AC compressor is engaged, but that's more a function of idle speed me thinks. If this car had an accelerator cable, we'd not be having these discussions I believe. lol.

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2002 X5 4.6 IS (Imola on black)
2001 740i M-sport (Alpine on black)
2000 540it M-sport (Titanium on grey)
1995 540i/6 (Hellrotted like a MMM)