Vintage Cars: 77 W123 In gear rattling, vacuum leaks, head banger

Question
she purrs like a champ in neutral and park...but in gear, she starts retailing like a head banger at a heavy metal concert..should i mess with the idle controller on the instrument panel, maybe mess with the idler assembly under the hood?

thanks for your input

Jason

Answer
do the RPMS drop dramatically?  If that is the case then adjust idle linkage.  IF it is missing , then check timing and if OK - fuel delivery /air intake ( rule out vacuum leaks) .  I am assuming this is a gas car and not diesel
If it is a diesel , then may be fuel delivery ( pump)


so she is a diesel, could be a host of issues, if she lacks power, check valves, look for vacuum leaks and ensure injectors are properly working.
Also , may not be mechanical at all but  the  motor mounts, if they are trashed that may be the issue