I'm going to multiclass as a Rogue Warlock, and I thought it'd be fun to use Eldritch Blast (Eldritch Spear) as a sort of sniper weapon. Is there some ability/item that allows you to silence spells, or are most spells like EB quiet enough to be stealthy?
Eldritch Blast's (EB) description states "A beam of crackling energy streaks toward a creature within range. [...]", so I would say it's at least audible in the near vicinity (the next room?). You could place a Silence spell on a creature before casting EB to absolutely silence the spell, as well as cries for help of your victim.
Unfortunately it seems there are no standard items to gain access to more castings of Silence as per this question unless you count the very generic Ring of Spell Storing, but you could talk to your DM about homebrewing an item or how s/he would rule how far the sound of EB travels. It's no Thunderclap at least, that's for sure.
EDIT: since the title of the question has been modified, I should add the description of Silence (emphasis mine): "For the Duration, no sound can be created within or pass through a 20-foot-radius Sphere centered on a point you choose within range. Any creature or object entirely inside the Sphere is immune to thunder damage, and creatures are Deafened while entirely inside it. Casting a Spell that includes a verbal component is impossible there."
Now, there are two low-level "loud" spells that were mentioned already, Thunderclap (Cantrip) and Thunderwave (1st level). These both have a range of self with 5 and 15 feet radius, respectively. Thunderclap only has Somatic components, while Thunderwave has Vocal and Somatic components. This means that while you could cast Thunderclap inside a area of Silence, all creatures within would be immune to its damage. You cannot cast Thunderwave inside of Silence because of the need for Verbal components (unless you can forego that e.g. with the Sorcerer's Subtle Spell metamagic), but here as well the creatures within Silence's area would be immune to the damage.