another idea
Change your oil and mix in one quart of sythetic, same viscosity as the rest of the oil. The syn will start the cleaning action without it being too severe. Keep an eye on your oil and if it gets dirty faster than normal, change both it and the filter and run another quart of syn. If it was buildup in the lifters, they should quiet down within a few hundred miles.
The seaform trick is an engine flush, you add an entire can to the engine oil just before an oil change, drive for a few miles or a day or two and change it. But as other posters have said, this can losen big chunks of crud which can cause issues. The same thing can happen if you switch to synthetic oil entirely.
How is your oil pressure? If it's starting to drop you might try a higher viscosity oil, the lifter clattering could be due to low oil pressure. So something else to try -