Check out which companies are making headlines before the bell:

CBS – CBS is considering an all-stock bid for Viacom that would value Viacom below its current market value, according to sources. The bid would propose that CBS CEO Leslie Moonves would stay at the combined company for at least two years. There is no word yet on the potential timing of any bid.

Tesla – Tesla remains on watch after slumping more than five percent in Monday trading, bring its 2018 slide to about 19 percent. The automaker said CEO Elon Musk is focusing his efforts on production of the Model 3, with the latest production figures set for release sometime this week.

21st Century Fox – Fox is mulling the separation of Sky News from parent Sky to address regulator concerns. Fox is trying to buy the 61 percent of Sky that it does not already own.

Spotify – Spotify begins trading today in the New York Stock Exchange in a direct listing rather than an initial public offering. The New York Stock Exchange has set a reference price for the music streaming service's stock at $132 per share.

Boeing – Boeing's efforts to forge a joint venture or a possible with Brazil's Embraer is running into resistance from labor union leaders in Brazil. Officials are complaining that a deal is being negotiated by the government behind their backs, with no information about why such a transaction would be favorable.

Overstock.com – Overstock pulled a secondary stock offering, following a slump in the online retailer's shares which have pushed the stock down more than 60 percent this year. Overstock cited market volatility and the stock's price for terminating the offering.

Walmart – The retailer announced an international money transfer service called Walmart2World, in a partnership with U.S.-based payments company MoneyGram International.

Constellation Brands – Constellation was downgraded to "hold" from "buy" at Stifel Nicolaus, which cited valuation after the spirits producer's shares ran up 41 percent over the past 12 months. The firm also cut its price target on the stock to $228 from $245.

GameStop – GameStop was downgraded to "hold" from "buy" at Loop Capital Markets, which pointed to a lack of positive catalysts for the videogame retailer. Loop also cut its price target at GameStop shares to $14 from $26.

General Motors – The automaker will no longer issue monthly sales figures instead releasing them quarterly. GM said a one-month period is too short to paint an accurate picture of overall sales trends.

Schlumberger – The oilfield services company was upgraded to "buy" from "hold" at SunTrust Robinson Humphrey, which thinks a recent pullback in the stock represents an attractive buying opportunity.

Hibbett Sports – The athletic apparel and footwear retailer got a reiterated "buy" recommendation from Canaccord Genuity following a meeting with management. The firm said it is convinced that a recovery is on track following that meeting, and that there are plenty of tailwinds to drive earnings past current consensus in the second half of the year.