[svenfendersen]

Hi, folks-

My first post here! I have a 2013 FR-S 6MT, with ~15k miles. I adore it. However, I am hearing an odd engine noise, typically when I rev match on a downshift and the engine revs from ~2500 to 3250 RPM, and the noise seems to occur right as the engine revs past 2800 RPM. It's a metallic chatter/rattle/lashing sound that happens for a fraction of a second as I go from off throttle to a quick on-throttle blip (with clutch engaged, so motor is 'unloaded' for a rev match), but sometimes I hear it in gear under load if I alternate off-throttle to half-throttle (or more) around 2800 RPM. It sounds pretty much just like my old M3 did when it had the dreaded VANOS issues, but a little less dramatic. Here are a few other factors:
1) Does not happen every time I drive it, but I hear it once every few days
2) Only occurs when the car is warm (more than 5 mins of driving)
3) No CEL/codes or anything
4) Engine does not stumble or surge when the noise is made. No appreciable loss of power when I hear it when the car is in gear.
5) Car is totally stock except for a Perrin cat back exhaust.
6) I run Shell gas, 92 octane (highest octane I can seem to find in the seattle area, sadly)

I've been looking for a thread about it on here and the closest issues I seem to come across are folks that have more sustained rattle sounds when completely off throttle slowing to a stop.

To me it kind of sounds like a timing chain could be slapping against it's cover, or the chain guides, as a result of sudden tension as I go from off-throttle to on-throttle. Maybe a loose tensioner? I was thinking maybe it could also be a loose heat shield, but I haven't had a chance to investigate that much. I figure I'll have the dealer check it out, but wanted to see if anybody on here has heard similar sounds. Any insights on this would be awesome... thanks!