1997 Thunderbird - strong reapeted knocks - rear left

I have a 1997 Ford Thunderbird with 139,000 miles.
I have the most annoying problem I ever had with a car. Every time I accelerate above 50 mph or turn to the right, I hear strong knocks that gradually increase as I accelerate or sharpen my turn. I think that the left side of the fuel tank moves and knocks on the cars frame.
Is anyone had something like this?
is that a must for replacing the fuel tank?
Will securing the right side of the fuel tank or adding isolation material between them will help?
I will thank any advise