I must've missed your first post which explained why you needed a new recoil spring assembly from Glock ...
Don't take this as gospel, and you should also ask this question over on the Glock Talk forum, as there are a lot of G36 owners over there ... However ...
What you MAY be feeling during manual slide retraction is the rubbing of the recoil spring on the bottom of the barrel. The next time you remove the slide take a look at the bottom of the barrel, along the "center" of it, lengthwise. If you see any rub marks, that's more than likely what you're hearing and feeling. This is considered normal by most Glock owners, and has been reported by many of the large caliber Glock owners and shooters over on GT. As long as it isn't out of the ordinary, and doesn't actually interfere with the normal operation of the pistol, you're probably just going to have to ignore it. Lubricating the entire external circumference of the barrel is always a good idea ... especially the bottom of the barrel, which might help lessen this noise. This issue has been raised by some G21 owners over on the GT forum ...
Also, it's not uncommon for the manual operation of a pistol's slide to feel differently ... less "smooth" ... than it actually operates during shooting. You ought to be able to manipulate it by hand, though, and if you can't the pistol should be checked out by a Glock armorer or factory technician.
Going to a heavier recoil assembly may create the potential for "limp-wristing" issues in a polymer framed pistol, depending on the shooter, as well as affect the ejection characteristics of your pistol. While I've installed heavier recoil springs in some pistols, I've done it for specific reasons. I've also installed heavier magazine springs to compensate for the faster slide velocity during counter recoil caused by the heavier recoil springs, as well, so the velocity of the slide doesn't "over-come" the strength of the magazine springs.
A compact & subcompact pistol, with a short recoil cycle, can often be more sensitive to slide mass & slide velocity issues, which are sometimes complicated by a shooter's grip, and increasing the recoil spring strength can sometimes create a new problem where one didn't exist previously ...
Unless you have a reason to believe the pistol is unsafe to fire ... in which case it should examined by an armorer or the factory technicians ... if it works when you shoot it normally, I'd leave it alone and enjoy it.
The great folks at Wolff Company can certainly discuss this issue with you, as well ...
Also, keep in mind that shooting "under powered" ammunition has caused malfunctions in more pistols than just Glocks ... Shoot good QUALITY ammunition in your G36 ... and while I personally don't favor +P ammunition in short barreled pistols due to recoil control/recovery, you can get a lot of input about using +P in G36's from a lot of the owners, as well as asking the factory about using +P ...
