diff --git a/index.d.ts b/index.d.ts
index 9ac5b80f5..2ba3adcc0 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -2,11 +2,10 @@
 
 import { EventEmitter } from 'events';
 import { SecureContextOptions } from 'tls';
-import Transport from './lib/Transport';
+import Transport, { ApiResponse, EventMeta, SniffMeta } from './lib/Transport';
 import Connection, { AgentOptions } from './lib/Connection';
 import ConnectionPool, { nodeSelectorFn, nodeFilterFn } from './lib/ConnectionPool';
 import Serializer from './lib/Serializer';
-import { ApiResponse } from './lib/Transport';
 
 declare type anyObject = {
     [key: string]: any;
@@ -471,4 +470,14 @@ declare const events: {
     SNIFF: string;
 };
 
-export { Client, Transport, ConnectionPool, Connection, Serializer, events, ApiResponse };
+export {
+  Client,
+  Transport,
+  ConnectionPool,
+  Connection,
+  Serializer,
+  events,
+  ApiResponse,
+  EventMeta,
+  SniffMeta
+};
diff --git a/lib/Transport.d.ts b/lib/Transport.d.ts
index 8839ce3eb..91099291f 100644
--- a/lib/Transport.d.ts
+++ b/lib/Transport.d.ts
@@ -6,42 +6,61 @@ declare type noopFn = (...args: any[]) => void;
 declare type emitFn = (event: string | symbol, ...args: any[]) => boolean;
 
 interface TransportOptions {
-    emit: emitFn & noopFn;
-    connectionPool: ConnectionPool;
-    serializer: Serializer;
-    maxRetries: number;
-    requestTimeout: number | string;
-    suggestCompression: boolean;
-    sniffInterval: number;
-    sniffOnConnectionFault: boolean;
-    sniffEndpoint: string;
-    sniffOnStart: boolean;
+  emit: emitFn & noopFn;
+  connectionPool: ConnectionPool;
+  serializer: Serializer;
+  maxRetries: number;
+  requestTimeout: number | string;
+  suggestCompression: boolean;
+  sniffInterval: number;
+  sniffOnConnectionFault: boolean;
+  sniffEndpoint: string;
+  sniffOnStart: boolean;
 }
 
 export interface ApiResponse {
-    body: any;
-    statusCode: number | null;
-    headers: any;
-    warnings: any[] | null;
+  body: any;
+  statusCode: number | null;
+  headers: any;
+  warnings: any[] | null;
+}
+
+export interface EventMeta {
+  connection: Connection;
+  request: any;
+  response: ApiResponse;
+  attempts: number;
+  aborted: boolean;
+}
+
+export interface SniffMeta {
+  hosts: any[];
+  reason: string;
 }
 
 export default class Transport {
-    emit: emitFn & noopFn;
-    connectionPool: ConnectionPool;
-    serializer: Serializer;
-    maxRetries: number;
-    requestTimeout: number;
-    suggestCompression: boolean;
-    sniffInterval: number;
-    sniffOnConnectionFault: boolean;
-    sniffEndpoint: string;
-    _sniffEnabled: boolean;
-    _nextSniff: number;
-    _isSniffing: boolean;
-    constructor(opts: TransportOptions);
-    request(params: any, callback: (err: Error | null, result: ApiResponse) => void): any;
-    getConnection(): Connection | null;
-    sniff(callback?: (...args: any[]) => void): void;
+  static sniffReasons: {
+      SNIFF_ON_START: string;
+      SNIFF_INTERVAL: string;
+      SNIFF_ON_CONNECTION_FAULT: string;
+      DEFAULT: string;
+  };
+  emit: emitFn & noopFn;
+  connectionPool: ConnectionPool;
+  serializer: Serializer;
+  maxRetries: number;
+  requestTimeout: number;
+  suggestCompression: boolean;
+  sniffInterval: number;
+  sniffOnConnectionFault: boolean;
+  sniffEndpoint: string;
+  _sniffEnabled: boolean;
+  _nextSniff: number;
+  _isSniffing: boolean;
+  constructor(opts: TransportOptions);
+  request(params: any, callback: (err: Error | null, result: ApiResponse) => void): any;
+  getConnection(): Connection | null;
+  sniff(callback?: (...args: any[]) => void): void;
 }
 
 export {};
diff --git a/lib/Transport.js b/lib/Transport.js
index 941e17c8f..51f29667b 100644
--- a/lib/Transport.js
+++ b/lib/Transport.js
@@ -29,7 +29,7 @@ class Transport {
     this._isSniffing = false
 
     if (opts.sniffOnStart === true) {
-      this.sniff()
+      this.sniff(Transport.sniffReasons.SNIFF_ON_START)
     }
   }
 
@@ -112,7 +112,7 @@ class Transport {
         this.connectionPool.markDead(meta.connection)
 
         if (this.sniffOnConnectionFault === true) {
-          this.sniff()
+          this.sniff(Transport.sniffReasons.SNIFF_ON_CONNECTION_FAULT)
         }
 
         // retry logic
@@ -225,19 +225,22 @@ class Transport {
   getConnection () {
     const now = Date.now()
     if (this._sniffEnabled === true && now > this._nextSniff) {
-      this.sniff()
+      this.sniff(Transport.sniffReasons.SNIFF_INTERVAL)
     }
     this.connectionPool.resurrect(now)
     return this.connectionPool.getConnection()
   }
 
-  // TODO: add sniff reason
-  //       'connection-fault', 'interval', 'start', ...
-  sniff (callback = noop) {
+  sniff (reason = Transport.sniffReasons.DEFAULT, callback = noop) {
     if (this._isSniffing === true) return
     this._isSniffing = true
     debug('Started sniffing request')
 
+    if (typeof reason === 'function') {
+      callback = reason
+      reason = Transport.sniffReasons.DEFAULT
+    }
+
     const request = {
       method: 'GET',
       path: this.sniffEndpoint
@@ -251,7 +254,7 @@ class Transport {
 
       if (err != null) {
         debug('Sniffing errored', err)
-        this.emit('sniff', err, null)
+        this.emit('sniff', err, { hosts: [], reason })
         return callback(err)
       }
 
@@ -259,12 +262,20 @@ class Transport {
       const hosts = this.connectionPool.nodesToHost(result.body.nodes)
       this.connectionPool.update(hosts)
 
-      this.emit('sniff', null, hosts)
+      this.emit('sniff', null, { hosts, reason })
       callback(null, hosts)
     })
   }
 }
 
+Transport.sniffReasons = {
+  SNIFF_ON_START: 'sniff-on-start',
+  SNIFF_INTERVAL: 'sniff-interval',
+  SNIFF_ON_CONNECTION_FAULT: 'sniff-on-connection-fault',
+  // TODO: find a better name
+  DEFAULT: 'default'
+}
+
 function toMs (time) {
   if (typeof time === 'string') {
     return ms(time)