More information about the item:
You can use the item without breaking stealth. However, interacting with Griftah will cancel stealth.
Griftah will stay for approximately 1 minute after being summoned, then sneaks away and despawns. An emote will appear: "Good seein' ya." (Possibly bug or typo - Griftah's name is left off.)
Dingy Iron Coins are used for the rogue-only weekly quest - also offered by Griftah - Da Big Score. It's worth 250 gold.
The next quest is If You're Sure. PLEASE NOTE THAT THIS IS ONLY FOR DUMPING EXCESS. It's only worth 10 gold. Just wait if you can till the next week to complete Da Big Score again.
Secretive Whistle - Griftah's conversion rate
= 5 Dingy Iron Coins Diamond Ring
= 4 Emerald Ring
= 3 Glistening Ring
= 6 Sapphire Ring
= 2 Slimy Ring
= 10 Opal Amulet
= 4 Oozing Amulet
= 8 Ruby Amulet
= 6 Sparkling Amulet
= 12 Chain of Hopes
= 18 Choker of Nightmares
= 6 Locket of Dreams
= 20 Glowing Ancestral Idol
= 30 Magma-Infused War Beads