Check your Y-pipe at the cylinders and for cracks in the Y part.
If it's loose you need to use "nord-lok" washers in place of the lock washers that's on it .
You can get them at most fastener stores.
If it's cracked then you need to replace it of have it welded.
Another thing it could be is the y- pipe is leaking where it connects to the pipe ....
You can take and bend the loops out some to make the springs pull a bit tighter to hold it tight.
If the springs are against the pipe it could be binding when it flexes...reason for above suggestion.