Help With Noise - Thud/wump Sound After Engine Started

[sa_2003]

Hello to all, new owner of 99 year rx300, 140k km. Full service history, one owner prior, bought from Lexus dealer (interestingly, given popular forum topics, with rebuilt warrantied transmission)

There is a thud/wump sound inconsistently but once only after each startup - ? HVAC related, seems on the otherside of the firewall from driver. is there an air intake damper or such back there that can stick? All systems working normally.

Thanks for any insight.

Cheers.