I am having the same symptoms as you are with my 87 260E. I have tried retarding the timing, different premium fuels (Shell, Chevron, Arco and Union 76), octane boosters and still can't get rid of my noise.
It doesn't seem like I have any power loss either. It is just a annoying sound while driving.
If I solve my problem or you solve yours, please let me know and I will do the same.
1987 260E Charcoal Grey 157,000 Miles
1987 260E Forest Green 120,000 Miles