Ls430 exhaust drone

[Stevofoto]

Hi everyone, my first post on here, although I’ve been on here reading plenty of threads before now but still can’t find a constructive solution to my problem! 

Ive had my current ls430 just over a year and it’s a dream but in order to get it through its last mot I needed to get the rear back boxes welded up due to leaks. 

All passed and I thought all was good but now there is an awful droning noise from underneath the car that just fills the whole cabin with noise especially speeds higher than 40mph. 

Took it back to get looked at as I was sure the exhaust wasn’t fitted back together properly but all looks ok. Went to another garage who confirmed this but they also test drove it and could hear the noise but couldn’t pinpoint the problem. 

Weeks later I’ve picked up a used good condition centre silencers and back boxes and had them fitted yesterday and alas the problem is still there. If nothing else I think it’s worse. 

I miss my quiet 430 and wondered if anyone had any experience of this. I don’t fancy a custom exhaust build or paying ££££ Lexus themselves. 

I thank you guys in advance and looking forward to any replies. Sorry guys just noticed I’ve put this in the wrong part of forum but can’t delete it on my phone  

Cheers

[bobjol]

Check if one of the back boxes is very close to the fuel tank edge. On inspection it may look ok,  but while driving there's a possibility that it will come into contact with the fuel tank and create the drone noise through the car.