I had a very similar problem, turned out the exhaust was touching the metal support strap near the front heatshield under braking. The only reason I can see for this is wear in the engine mounts allowing the engine to pivot under load.
I put spacers under the strap to move it away from the exhaust, this has cured the problem until I get round to changing the engine mounts.
I put spacers under the strap to move it away from the exhaust, this has cured the problem until I get round to changing the engine mounts.