EJG87478

I have a 2000 ES 300 with 69,XXX miles on it. I 've been having problems for some time with a constant deep rhythmic chugging noise that seems to be coming from underneath the vehicle. The noise is most prominent whenever I am stopped in drive and especially whenever there are abrupt humidity changes or if temperatures outside are roughly less than 80F. However, I don't detect any of the noise or vibrations when I'm at speed. At it's worse the noise is accompanied by some vibrations that seem to course through the vehicle's frame in time with the chugging. Any idea what all this might be about? Thanks.