I have a number of suppressed .22 LR guns, as well as .22 cal PCPs. I fire them both indoors, and with the right ammunition I can certainly say there is not much difference at all between them. Certainly, on an equivalent power basis (i.e. with both shooting in the 85-95 FPE range) there is no difference.
If you use the CCI suppressor rounds in a suppressed rifle then you can expect it to be very quiet. The loudest sound will be the cycling of the action if you are using something like a Ruger 10/22. Even that can be improved by switching out the bolt stop from a steel piece to a delrin rod. I hear no difference shooting that suppressed (with a Q Erector suppressor) as compared to my FX Impact or Crown in .30 with a DonnyFL suppressor, and the same was true of my Benjamin Marauder .22, which I did not additionally suppress. Usiing the Q Erector suppressor on my Savage bolt action .22 is the quietest of all, since all you hear is really the click of the striker hitting the primer, and the sound of the bullet hitting the backstop.
So, from my experience, you will not gain anything in terms of quietness using a PCPO like a Marauder over a suppressed .22LR, firing CCI suppressor rounds. Note that I have found that the CCI Quiet .22 rounds will not cycle my Ruger 10/22s. They work fine in the bolt gun however, but have a lot lower FPE. The Marauder will also not make anything like the power of a suppressed .22LR, as pointed out above by AirArcher66.
The Marauder WILL be quieter than a suppressed .22LR pistol. I have one Volquartsen .22 pistol that I shoot suppressed, and also two Rugers, a MK II and a MK IV. All three are substantially louder than the rifles when suppressed, but still hearing safe. Its just that at they point they are marginally louder than my .22 air rifles, including the Marauder.
By the way, here is the data I got at the range a little while ago about the performance of the CCI rounds. You will struggle to get that kind of power from any .22 air rifle and certainly not from a Marauder. For my purposes, I don’t fire the .22LR guns in my yard, because they have too much power and there is always the risk fo the bullets traveling off property. This is NOT the case with Air rifles, especially if you are shooting pellets, so almost all my home shooting is done with pellets.
.22LR
HW MW High Low Avg FPE ES SD
Ruger MK IV 939 895 916 83.8 4.8% 15.4 CCI Supp 45gr [03/27/22]
Ruger MK 2 986 963 974 84.2 2.4% 7.7 CCI SV 40gr [03/27/22]
Ruger 10/22 915 83.7 CCI Supp 45gr [03/22/22]
Ruger 10/22 1030 94.2 CCI SV 40gr [03/27/22]
.22 CAL AIR RIFLES (25.4 GR PELLETS)
Crown Mk2 500 Max Med 865 858 862 41.9 0.8% 2.0 [10/23/21]
Huben K1 1014 1002 1008 57.3 1.2% 4.6 [03/14/22] REG 16.5 PW 25
34 GRAIN PELLETS
Crown Mk2 700 Max Max 872 854 861 56.0 2.1% 7.6 [05/15/21]
Huben K1 946 921 932 65.6 2.7% 7.1 [03/28/22] REG 16.5 PW 30