Hello boblexus430,
I have the exactly same problem on my 2003 LS430. There was a leak on the passenger side behide the O2 sensor just inside the heat shield. I took it to local repair shop. They cut out the leaking section and welded a new pipe for the area. It cost only $80. But the heat shield was taking off. Anyway, it's a quick fix for the leaking and noise.