HW80

To get rid of twang, you can fit a kit that has guides for the spring.
Tin Bum tuning sell them.
If you take the gun apart and can measure accurately the spring ID and the ID of the piston, it's pretty easy to make up a top hat spring guide for inside the piston. Then using some coke can, or coke bottle make a sleeve for the spring. These take up the gaps around the spring and absorb vibration that causes the twang.

https://www.gatewaytoairguns.org/GTA/index.php?topic=103192.0

Bit of info here on what ppl have done.

Need Mike to comment, he is an ace at problem solving. Did it have the spring guides? I forgot to put them back in on a gun after a rebuild and it's sounded like a church bell after each shot.... Another option to try is put a washer in behind the spring to give it more tension/pressure (some people call the power washer etc...)

From what I understand it does. I've greased the spring on the gamo, just a light smear around spring. No lumps or clumps.
piston as well.
TX will get it soon as its just started that twang sound. As I haven't touched it since I brought it, I'm guessing it's due.

AIR GUNS  FTPs
	Low power	Ft Pounds		Hi Power	Ft Pounds
HW45	335			430	6.2
177    7.87g	310			418	5.8
	322.5	3.5		419	6
HW80
22    14.35				760
				760	18.4
TX200				865
177     7.87g				863
				864	13.4
FX Wildcat	level 4			level 7
15.8g	750	20		930	30
22   21g slug				800	30